A contract is a legally enforceable agreement between two or more parties who exchange promises or performances of value. Under the Restatement (Second) of Contracts, an agreement becomes a contract when three things line up: mutual assent (both sides actually agree), consideration (both sides give something up), and legal capacity (both sides are competent to make the deal). Miss any one of them and what you have is a conversation, a gift, or a dispute waiting to happen, but not a contract a court will enforce.
The Three Elements That Make an Agreement a Contract
Everything else in contract law builds on these three. If you can identify them in your situation, you likely have a binding deal. If one is missing, you probably don’t.
Mutual Assent: Offer and Acceptance
Mutual assent is the formal name for both sides agreeing to the same thing. The Restatement (Second) of Contracts § 17 puts it plainly: formation requires “a manifestation of mutual assent to the exchange and a consideration.”1OpenCasebook. Restatement of Contracts Second 3, 17, 18, 22, 23, 24 In practice, that means one party makes an offer and the other accepts it.
A valid offer shows a present intent to be bound and contains terms definite enough for a court to figure out what was promised. “I might sell you my car someday” is not an offer. “I’ll sell you my 2020 Honda Accord for $15,000, with delivery next Friday” is. The person making the offer is the offeror; the person receiving it is the offeree.
Acceptance has to match the offer’s terms exactly. Lawyers call this the mirror image rule. Change anything and you haven’t accepted the offer, you’ve killed it and made a new one. Offer a laptop for $800, get back “I’ll take it for $700,” and the original offer is gone. The response is a counteroffer, and now the original offeror decides whether to accept or reject.
Timing can matter too. Under the mailbox rule, acceptance takes effect the moment it is sent, not when the offeror receives it. Drop an acceptance letter in the mail on Monday and the contract forms Monday, even if the other side doesn’t read it until Wednesday. Option contracts and electronic communications often follow different timing rules, and the offer itself can specify how and when acceptance must happen.
One place this trips people up: advertisements. A store ad reading “TVs on sale for $299” is almost never a binding offer. The law treats it as an invitation to negotiate. The store is inviting you to come in and make an offer to buy, and the store can refuse to sell. Rare exceptions exist for ads with extremely specific terms and a clear limit on who can accept, but those cases are unusual.
Consideration: Something for Something
An agreement where only one side gives something up is a gift, not a contract. Consideration is the legal term for the exchange that separates the two. Each party must experience what lawyers call a “legal detriment,” meaning they promise to do something they weren’t already required to do, or they agree not to do something they had every right to do.
The exchange can take many forms. Paying money is the obvious one, but agreeing to provide a service, transferring property, or even giving up the right to file a lawsuit all qualify. What doesn’t matter is whether the deal was fair. Courts look at whether an exchange happened, not whether the price was right. A contract to buy a car worth $20,000 for $5,000 is still enforceable if both sides genuinely bargained for it. Even a token payment can work in the right circumstances, though courts get suspicious when a nominal amount is just a fig leaf over what is really a gift.
The one scenario where courts will step in on fairness is unconscionability, where the terms are so lopsided they “shock the conscience.” That is a high bar, and it usually requires both an unfair process (one side had no real ability to negotiate) and unfair terms (the deal itself is wildly one-sided).
Capacity and a Lawful Purpose
Even with a clear offer, acceptance, and consideration, a contract fails if either party lacks the legal capacity to enter it or if the deal itself is illegal.
Capacity means being able to understand what you are agreeing to. Two groups commonly lack it: minors and people with certain mental impairments. The age of majority is 18 in most of the country, and contracts signed by minors are generally voidable at the minor’s option. The minor can walk away; the adult on the other side cannot. People under legal guardianship or experiencing a mental condition that prevents them from understanding the agreement’s terms and consequences get similar protection.
The agreement must also serve a lawful purpose. A contract to sell illegal drugs, fix prices, or commit fraud is void from the start. No court will enforce it, and neither party can sue the other for failing to hold up their end. This extends to agreements that violate public policy even when no specific statute is broken. An employment contract waiving all workplace safety protections, for example, could be struck down on public policy grounds.
Written or Spoken: When Form Matters
Plenty of contracts are perfectly enforceable as spoken agreements. Hire a neighbor to mow your lawn for $50, and when the work is done the oral deal binds both of you. Evidence of these agreements typically comes from witness testimony or the parties’ own conduct.
Certain categories, however, must be in writing under a rule called the Statute of Frauds. The required writing does not have to be a formal contract with a lawyer’s signature block. It just needs to show that an agreement existed, identify the parties, outline the key terms, and bear the signature of the person you are trying to hold to the deal. The categories that typically require a writing include:
- Real property transfers, including sales, leases, mortgages, and easements involving land
- Agreements that by their terms cannot be fully performed within 12 months from the date they are made
- Sales of goods worth $500 or more, a threshold that comes from the Uniform Commercial Code, which most states have adopted2Legal Information Institute. Uniform Commercial Code 2-201 – Formal Requirements; Statute of Frauds
- Promises to pay someone else’s debt, known as suretyship agreements
- Agreements made in consideration of marriage, such as prenuptial agreements
An oral agreement in one of these categories is not necessarily dead. The partial performance exception can save it, particularly for real estate. If you paid part of the purchase price, took possession of the property, and made improvements to it, a court may enforce the oral deal despite the absence of a writing. Those actions serve as evidence that a real agreement existed, because people don’t typically hand over money and renovate property they have no claim to.
Federal law has also kept pace with how people actually make deals. Under the Electronic Signatures in Global and National Commerce Act, a contract or signature cannot be denied legal effect simply because it exists in electronic form.3Office of the Law Revision Counsel. 15 USC 7001 – General Rule of Validity Clicking “I agree,” typing a name in a signature field, or using a platform like DocuSign all count. The key requirements are that both parties intended to sign, both consented to conducting business electronically, the system links the signature to the record, and the record can be stored and reproduced later. Wills, certain trusts, powers of attorney, and court orders are often excluded from electronic signature laws depending on the jurisdiction, but for everyday contracts the electronic version carries the same legal weight as ink on paper.
Bilateral, Unilateral, Express, and Implied
The law recognizes several kinds of contracts depending on how the agreement was formed and what the parties promised. Knowing which kind you have helps you figure out when the deal became binding and what each side owes.
A bilateral contract is the most common type. Both parties exchange promises: you agree to paint my house, and I agree to pay you $3,000 when the job is done. Each side is bound the moment the promises are exchanged.
A unilateral contract works differently. One party makes a promise, but the other party accepts by performing an action rather than by making a return promise. The classic example is a reward poster: “I’ll pay $500 to whoever finds my dog.” Nobody is obligated to look for the dog, but if someone does find it and returns it, the person who posted the reward owes the money.
Contracts also break down by how they are communicated. An express contract spells out its terms, whether in writing or spoken aloud. An implied contract arises from behavior rather than words. When you sit down at a restaurant and order food, nobody signs anything, but both sides understand the deal: they cook, you pay. Courts enforce implied contracts the same way they enforce express ones, as long as the conduct clearly shows that both parties understood and intended the arrangement.
Void, Voidable, and Unenforceable
When something goes wrong at formation, the result is not always the same. These two terms sound similar but mean very different things.
A void contract never had any legal force. It is treated as if it never existed. Contracts for illegal purposes are void. Nobody can enforce them, and no action by either party can make them valid.
A voidable contract is valid and enforceable unless the protected party decides to cancel it. The minor who signed a car lease can choose to honor it or walk away. The person who signed under duress can choose to affirm the deal or void it. Until the protected party acts, the contract remains in effect. This distinction determines who holds the power: with a void contract, neither side can enforce anything; with a voidable one, the protected party controls whether the deal stands.
When a Promise Without Consideration Still Binds
Sometimes a promise that lacks consideration is still enforceable. If someone makes a promise they should reasonably expect you to rely on, and you do rely on it to your detriment, a court can enforce that promise under a doctrine called promissory estoppel. The Restatement (Second) of Contracts § 90 lays out three requirements: the promisor should have expected the promise to induce action, the promise actually did induce action, and enforcing it is the only way to avoid injustice.4OpenCasebook. Restatement Second of Contracts 90 – Promissory Estoppel
A common example: your employer promises you a pension if you stay for 20 years. You stay, but there is no formal contract and no consideration in the traditional sense. If the employer tries to back out, a court can enforce the promise because you gave up two decades of career alternatives based on it. The remedy in promissory estoppel cases can be smaller than full contract damages, limited to whatever justice requires.
What Breaking a Contract Costs
A valid contract is enforceable, and enforceability has teeth. A breach of contract occurs when one party fails to perform as promised, and the consequences depend on how serious the breach is and what the non-breaching party lost.
The most common remedy is compensatory damages, which aim to put the injured party in the financial position they would have been in if the contract had been performed. This includes direct losses like unpaid contract prices and lost profits, along with consequential damages covering foreseeable downstream losses, such as business the injured party lost because the breaching party didn’t deliver supplies on time.
Some contracts include a liquidated damages clause that pre-sets the penalty for breach. Courts enforce these clauses if the agreed amount was a reasonable estimate of potential harm at the time the contract was signed and actual damages would be hard to calculate. If the amount is wildly disproportionate to any real harm, the court may treat it as an unenforceable penalty.
When money is not enough, a court can order specific performance, forcing the breaching party to actually do what they promised. This remedy is reserved for situations where the subject of the contract is unique and cannot be replaced with a cash substitute. Real estate is the textbook example. A judge will not order specific performance for a generic supply contract where the same goods are available from someone else.
One thing the non-breaching party cannot do is sit back and let losses pile up. The duty to mitigate requires reasonable efforts to minimize harm. If a supplier breaks a contract to deliver materials at $10 per unit, the buyer needs to find another supplier at the best available price, not refuse to look and then demand full lost profits. Damages that could have been avoided through reasonable action are not recoverable.