By Emma Hudson
What a contract is, and what it is not
A contract is not the same thing as a promise, a plan, or a friendly understanding. People make promises every day that no court would enforce, such as a promise to call a friend tomorrow. A contract is different because the law treats it as a binding obligation: if one party does not perform, the other may ask a court for a remedy.
A written document headed “contract” is not automatically binding either. A court looks at what the parties actually agreed, whether each required element is present, and whether any defense applies. A signed paper that lacks consideration, was signed by someone without capacity, or calls for an unlawful act may still fail.
Contract law in the United States comes mainly from state law. Most rules developed through court decisions, known as common law, and are collected in summaries such as the Restatement (Second) of Contracts. Statutes add rules for particular transactions, above all the Uniform Commercial Code, usually called the UCC, which governs contracts for the sale of goods. The same facts can therefore lead to different results in different states.
This article explains contract law principles in general, not employment forms or job paperwork. The principles apply to sales, services, rentals, loans, licenses, and everyday consumer agreements alike. The same questions recur in each setting. Was there an offer? Was it accepted? Was something of value exchanged? Did both parties have capacity? Was the purpose lawful? Did both parties truly agree? If any answer is no, enforceability is in doubt.
Principle 1: Offer
Every contract begins with an offer: a proposal by one party, the offeror, to another party, the offeree, showing a willingness to enter a bargain on stated terms. The Cornell Legal Information Institute describes an offer as part of negotiations in which a party agrees to do or not do something in exchange for consideration, and explains that an offer must be stated and delivered in a way that would lead a reasonable person to expect a binding contract to arise from its acceptance.
Three features identify a real offer. First, the offeror must intend to be bound if the offer is accepted. A joke or a boast usually lacks that intent, and courts generally ask what a reasonable person in the offeree’s position would have understood. Second, the offer must be definite enough for a court to tell what was promised: a named car at a stated price to a named buyer is definite, while an offer to sell “something sometime” is not. Third, the offer must be communicated to the offeree, who cannot accept an offer never received.
An offer is not the same as an invitation to negotiate. An advertisement, a price list, or a statement such as “I am thinking about selling my bicycle” is often treated as an invitation to make an offer, and the responding party is usually the one making the offer. An offer does not stay open forever. The offeror may revoke it before acceptance, the offeree may reject it, a counteroffer normally counts as a rejection and creates a new offer, or the offer may expire at a stated deadline or after a reasonable time. Two common exceptions keep an offer open: an option contract, where the offeree has given consideration to keep the offer open for a set period, and a merchant’s firm offer for the sale of goods under the UCC, discussed later.
Principle 2: Acceptance
Acceptance is the offeree’s assent to the terms of the offer. Together, offer and acceptance form mutual assent, the agreement at the heart of a contract. The Cornell Legal Information Institute lists mutual assent, shown by offer and acceptance, as one of the basic elements required for an agreement to be a legally enforceable contract. Without acceptance, there is only a proposal.
Under the traditional common law mirror image rule, acceptance must match the offer exactly. If the offeree changes a term, adds a condition, or accepts only part of the deal, that response is not an acceptance but a counteroffer, and the original offer is rejected. This rule rewards careful reading: a reply that says “yes, but” is often a no in contract law, followed by a new proposal.
Acceptance can be shown by words or by conduct: signing an agreement, saying “I accept,” clicking an acceptance button, or starting the performance the offer requested, depending on what the offer required. In a bilateral contract, a promise is exchanged for a promise. In a unilateral contract, the offer can be accepted only by performance, such as a reward offer accepted by returning a lost item. Silence, on its own, is generally not acceptance. The offer may prescribe how acceptance must be given.
The UCC changes the mirror image rule for sales of goods. Under UCC section 2-207, a definite and seasonable expression of acceptance, or a written confirmation sent within a reasonable time, operates as an acceptance even though it states terms additional to or different from those offered or agreed upon, unless acceptance is expressly made conditional on assent to the additional or different terms. Additional terms are then treated as proposals for addition to the contract. Between merchants, they join the contract unless the offer limits acceptance to its own terms, they materially alter the contract, or objection is given within a reasonable time.
Principle 3: Consideration
Consideration is the value exchanged that makes a promise enforceable. The Cornell Legal Information Institute lists consideration, meaning something of value exchanged, as a basic element of an enforceable contract. In the classic formulation, consideration is a bargained for exchange: each party gives something, or promises to give something, to induce the other party’s promise. Money, goods, services, a promise to act, and a promise to refrain from acting can all count, provided they are bargained for.
Consideration does not have to be equal in value. Courts generally do not weigh whether a bargain was a good deal: a used bicycle sold for a small sum still involves an exchange, even if it would fetch more on the open market. What matters is that each side gave up something of legal value, such as money, property, labor, or a legal right. A promise to give a gift, with nothing asked or given in return, is generally not enforceable as a contract. Other doctrines, such as promissory estoppel where a party reasonably relied on a promise to their detriment, can sometimes provide a remedy even without consideration.
Three familiar problems test whether consideration is really present. A promise to do something a party is already legally required to do, known as a pre-existing duty, is generally not new consideration, so a promise to pay extra for performance already owed may fail unless an exception applies, such as a genuine dispute settled by the modification. An illusory promise, which leaves performance entirely to the promisor’s choice, such as “I will buy from you if I feel like it,” also generally fails, because nothing is truly promised. Past consideration, something already done before the promise was made, is generally not consideration for a later promise, because it was not bargained for in exchange for that promise.
Principle 4: Capacity
Capacity means the legal ability to enter a binding contract. The Cornell Legal Information Institute lists capacity, with examples such as minimum age and sound mind, as a basic element of an enforceable contract. The law presumes that most adults have capacity, but it protects people who cannot fairly be held to a bargain they did not understand.
Minors are the most common capacity example. In most states, a person under the age of majority, usually 18, can enter contracts, but many of those contracts are voidable at the minor’s choice, meaning the minor can generally choose to cancel and in many cases recover what was given, subject to rules about returning benefits received. Exceptions include contracts for necessities such as food, shelter, and medical care. Details, including how and when a minor must act to cancel, vary by state.
Mental capacity is the second major category. A person who, at the time of contracting, cannot understand the nature and consequences of the transaction because of a mental condition may lack capacity, and the contract may be voidable or void depending on the state and the severity of the condition. Intoxication affects capacity only in limited circumstances: a contract is not automatically void because a party had been drinking. A person under a court-ordered guardianship may also have limited or no capacity to contract, depending on the court’s order.
Principle 5: Legality
A contract must have a lawful purpose. The Cornell Legal Information Institute lists legality, meaning a lawful purpose, as a basic element of an enforceable contract. A contract to commit a crime, to defraud someone, or to do something the law forbids is not enforceable, even if offer, acceptance, consideration, and capacity are all present. A court will generally refuse to help either party carry out an unlawful bargain.
Legality goes beyond criminal conduct. Contracts that violate a statute, such as licensing rules or consumer protection rules, may be unenforceable in whole or in part, depending on the statute and the state’s approach. Contracts against public policy can also fail, for example agreements that unreasonably restrain trade or interfere with the administration of justice, such as a promise to withhold testimony. Courts do not treat every harsh term as against public policy, and they are often reluctant to strike down a bargain between sophisticated parties who negotiated freely.
Principle 6: Mutual assent and genuineness of assent
Mutual assent is sometimes listed as its own principle and sometimes folded into offer and acceptance. Either way, both parties must intend to be bound by the same essential terms. The traditional phrase is a meeting of the minds, but modern courts usually apply an objective test: what would each party’s words and conduct reasonably signal to the other party, rather than what each party secretly believed? A party cannot escape a clear bargain by later claiming a private reservation that was never communicated.
Even when offer and acceptance appear to match, assent may not be genuine. A mutual mistake about a basic fact central to the bargain, such as both parties believing a painting is an original when it is a copy, can in some cases allow the contract to be avoided. A unilateral mistake, where only one party is mistaken, is harder to use as a defense, unless the other party knew of the mistake. Misrepresentation, a false statement of material fact that induces the other party to contract, can make a contract voidable, and fraudulent misrepresentation can also support a claim for damages. Duress, improper pressure such as a threat of unlawful harm, and undue influence, unfair persuasion over a vulnerable party in a relationship of trust, can also undermine genuine assent. In each case, the remedy and proof required depend on the state and the facts.
When a contract must be in writing
Most contracts in the United States can be oral. A spoken agreement to mow a lawn for a stated price, accepted and performed, can be binding even though nothing was written down. The difficulty with oral contracts is proof, not validity: memories fade and terms are disputed. Writing is therefore useful even when it is not legally required.
Some contracts, however, must be in writing to be enforceable. The traditional list comes from the Statute of Frauds, a doctrine inherited from English law and enacted in state statutes with local variations. Common categories include contracts that cannot be performed within one year from their making, contracts for the sale of an interest in land, promises to answer for the debt of another person, and promises made in consideration of marriage. Exact categories, exceptions, and writing requirements vary by state, so check the statute in the state whose law governs the agreement.
For sales of goods, UCC section 2-201 states the rule directly. A contract for the sale of goods for the price of $500 or more is not enforceable by way of action or defense unless there is some writing sufficient to indicate that a contract for sale has been made between the parties and signed by the party against whom enforcement is sought or by an authorized agent or broker. The writing is not insufficient merely because it omits or incorrectly states a term agreed upon, but the contract is not enforceable beyond the quantity of goods shown in the writing. Exceptions include goods specially manufactured for the buyer, admissions in court proceedings, and goods for which payment has been made and accepted or which have been received and accepted. Between merchants, a written confirmation can also satisfy the rule unless the recipient objects in writing within ten days after receiving it.
Electronic contracts and signatures
Electronic agreements are now routine, from online purchases to software subscriptions. Federal law supports their validity. Under the Electronic Signatures in Global and National Commerce Act, known as the ESIGN Act and codified at 15 U.S.C. section 7001, a signature, contract, or other record relating to a transaction may not be denied legal effect, validity, or enforceability solely because it is in electronic form. A contract also may not be denied legal effect solely because an electronic signature or electronic record was used in its formation.
That rule does not make every click binding. The other principles in this article still apply: an electronic agreement still needs offer, acceptance, consideration, capacity, legality, and genuine assent. A website that hides its terms behind a link a user never sees, or that obtains assent through a confusing interface, can still face a challenge on assent grounds. State electronic transaction laws add their own rules about attribution and consent to transact electronically. Some document types are excluded from ESIGN or subject to special requirements, such as certain wills, family law documents, court filings, and specific consumer notices. Treat an electronic agreement like a paper one: read the terms, keep a copy, and confirm what counts as acceptance before signing.
Performance, breach, and remedies
Once a contract is formed, each party must perform as promised, by the time and in the manner the contract specifies, unless the contract or the law excuses it. A party who performs only in part, or who performs late or defectively, may be in breach, although the seriousness of the failure matters. A minor breach gives the other party a claim for damages but does not necessarily end the contract. A material breach is serious enough to excuse the other party’s further performance and to support termination in appropriate cases. Whether a breach is material depends on the benefit already received, whether the breach can be cured, and how much performance has already occurred.
Breach can also occur before performance is due. An anticipatory breach happens when a party clearly states or shows by conduct that they will not perform when performance is due, and the other party may then treat the contract as ended and seek a remedy. Performance can also be excused in some situations, such as when the other party’s prior material breach excuses further performance, when an agreed condition fails to occur, or when performance becomes impossible because of an unforeseen event that destroys the subject matter. These excuses are narrow.
When a breach occurs, the most common remedy is money damages. The usual goal is to put the nonbreaching party in the position they would have been in if the contract had been fully performed, so far as money can do so. Compensatory damages cover losses directly caused by the breach, including the value of the promised performance and, in appropriate cases, incidental costs and consequential losses reasonably foreseeable when the contract was made. Reliance damages reimburse expenses incurred in reliance on the contract, and restitution returns benefits conferred on the breaching party. Some contracts include a liquidated damages clause, fixing an agreed amount for breach in advance, generally enforced when it is a reasonable estimate of likely loss at the time of contracting and not a penalty. Punitive damages are generally not awarded for an ordinary breach.
Not every remedy is money. A court may order specific performance, requiring a party to perform as promised, but this is usually reserved for cases where money damages are inadequate, such as a contract for a unique item of property. A court may order rescission, cancelling the contract and seeking to restore the parties to their positions before contracting, often together with restitution. The nonbreaching party generally has a duty to mitigate, meaning they must take reasonable steps to limit losses after a breach. Failure to mitigate can reduce the damages recoverable.
Common variations by state
Because contract law is mostly state law, several variations are worth naming. First, the line between common law and the UCC matters in every sale of goods case. Common law governs services, real estate, and most other transactions, while UCC Article 2 governs sales of goods, with its own rules on formation, additional terms, firm offers, warranties, and remedies. Mixed transactions, including both goods and services, raise a threshold question about which body of law predominates, answered by looking at the main purpose of the deal.
Second, firm offers show the UCC at work. Under ordinary common law, an offer is generally revocable before acceptance unless consideration supports keeping it open. Under UCC section 2-205, an offer by a merchant to buy or sell goods in a signed writing which by its terms gives assurance that it will be held open is not revocable, for lack of consideration, during the time stated or, if no time is stated, for a reasonable time, but in no event may such period of irrevocability exceed three months. Any such term of assurance on a form supplied by the offeree must be separately signed by the offeror. The three month limit is a ceiling, not a default: a firm offer stating a shorter period is irrevocable only for that shorter period.
Third, the Statute of Frauds varies by state. States differ in which categories require writing, in the exceptions they recognize, and in how strictly they require a signature. A contract enforceable orally in one state may require a signed writing in another. Parties operating across state lines should confirm both which state’s law governs the contract, often through a choice of law clause, and what that state’s writing rules require.
Fourth, Louisiana stands apart in its legal tradition. Louisiana private law draws on a civil law tradition rather than the common law tradition shaping contract law in the other states, and its Civil Code organizes obligations and contracts differently. Some contract concepts, terms, and sources therefore differ in Louisiana, even though many commercial transactions there are also governed by UCC provisions Louisiana has adopted.
A practical checklist before you sign
First, identify the parties and the subject matter in plain words: who is promising what, to whom, and by when? Second, check the exchange: is each side giving or promising something of value, stated clearly? Third, confirm the essential terms. Price, quantity, quality, timing, delivery, and payment terms should be written down, even for a simple deal, because disputes most often arise over terms the parties assumed but never recorded. Fourth, look for hidden terms: renewal clauses, automatic charges, termination fees, and dispute resolution clauses can matter more than the headline price. Fifth, confirm the form: does the agreement need to be in writing in the governing state, and is it signed by the party who would need to be bound? Sixth, keep a copy of the signed agreement and of any later changes, with later changes in writing where the original agreement requires that form. Seventh, if the stakes are high, the terms are unclear, or the other party is pressing for a quick signature, pause and seek advice from a qualified professional before signing.
Frequently asked questions
What are the basic principles of contract law?
The basic principles are the elements courts use to test whether an agreement is a binding contract: mutual assent through offer and acceptance, consideration, capacity, and legality, plus any required form such as a signed writing for certain categories. No single element is enough on its own, and the absence of any required element can prevent enforcement.
Does a contract have to be in writing to be valid?
Usually no. Most contracts can be oral, but some categories must be in writing under the Statute of Frauds or under UCC section 2-201 for sales of goods at a price of $500 or more. Even when writing is not required, a written and signed record is advisable because it proves the terms and reduces disputes.
What is the difference between a void contract and a voidable contract?
A void contract has no legal effect from the start, such as a contract for an unlawful purpose. A voidable contract is valid until the party with the right to cancel chooses to do so, such as many contracts made by a minor. An unenforceable contract is a third category: it may have been formed, but a court will not enforce it, often because a writing requirement was not satisfied.
What counts as consideration?
Consideration is something of value bargained for and exchanged, such as money, goods, services, a promise to act, or a promise to refrain from acting. It does not have to be equal in value to what the other side gives, but a pure gift promise with nothing in return generally lacks consideration. A promise to do something already legally owed is also generally not new consideration, subject to exceptions that vary by transaction type and state.
Can I accept a contract by email or by clicking a button?
In many cases yes, provided the other principles are satisfied. Under the ESIGN Act, a contract may not be denied legal effect solely because it is in electronic form, and state electronic transaction laws support electronic records and signatures in many transactions. The click or email must still show genuine assent to clear terms, and some document types are excluded or subject to special requirements.
What happens if the other party breaches the contract?
A breach may entitle the nonbreaching party to a remedy, most commonly money damages aimed at putting them in the position they would have been in if the contract had been performed. In limited cases, a court may order specific performance, rescission with restitution, or an injunction. The nonbreaching party should also take reasonable steps to limit further losses, because failure to mitigate can reduce recoverable damages.
Do contract rules differ from state to state?
Yes. Contract law is mainly state law, and states differ on matters such as the Statute of Frauds, capacity rules for minors, modification rules, consumer protections, and the treatment of particular clauses. The UCC adds a shared framework for sales of goods, but states enact it with local variations. Louisiana also differs because its private law draws on a civil law tradition.
A note on general information
This article explains general principles of United States contract law for readers who want to understand how contracts work. It is not legal advice for any person’s situation, and it does not create a lawyer-client relationship. Contract outcomes depend on the exact terms, the facts, the governing state law, and the evidence available, so a reader facing a real dispute, a high-value agreement, or an unclear term should speak with a qualified legal professional in the relevant state before acting. No outcome is promised or guaranteed by anything in this article.
Sources & references
- Cornell Legal Information Institute, Wex, "contract": basic elements of a legally enforceable contract, including mutual assent, consideration, capacity, and legality. https://www.law.cornell.edu/wex/contract
- Cornell Legal Information Institute, Wex, "offer": definition of an offer, revocation before acceptance, rejection, and counteroffer. https://www.law.cornell.edu/wex/offer
- Cornell Legal Information Institute, Uniform Commercial Code section 2-201, "Formal Requirements; Statute of Frauds": $500 writing threshold for sales of goods and related exceptions. https://www.law.cornell.edu/ucc/2/2-201
- Cornell Legal Information Institute, Uniform Commercial Code section 2-205, "Firm Offers": merchant firm offer irrevocability and three month ceiling. https://www.law.cornell.edu/ucc/2/2-205
- Cornell Legal Information Institute, Uniform Commercial Code section 2-207, "Additional Terms in Acceptance or Confirmation": acceptance with additional or different terms and the treatment of such terms between merchants. https://www.law.cornell.edu/ucc/2/2-207
- Cornell Legal Information Institute, 15 U.S.C. section 7001, Electronic Signatures in Global and National Commerce Act, general rule of validity: a record or signature may not be denied legal effect solely because it is in electronic form. https://www.law.cornell.edu/uscode/text/15/7001
