The types of contracts in law are usually sorted along four overlapping axes: how the agreement forms, what each party promises in exchange, how much of the deal has been performed, and whether a court will actually enforce it. A single agreement often wears several labels at once. A signed one-year apartment lease, for instance, is express, bilateral, executory, and valid all at the same time. Each label answers a different legal question, so knowing the vocabulary helps you figure out which rules apply to your situation.
What Makes Any Agreement a Contract
Before the labels mean anything, the agreement has to be a contract in the first place. Five ingredients are required: an offer, acceptance of that offer, consideration, legal capacity on both sides, and a lawful purpose.
Consideration is the piece people miss. Each side has to give up something of value in exchange for what the other side gives. A promise or performance counts as consideration only if it was bargained for, meaning sought by one party in exchange for the other’s promise or action.1Open Casebook. Restatement Second Contracts 71 – Consideration Money, services, a promise to act, even a promise not to do something you had the right to do — all of these work. A one-sided gift promise does not. “I’ll give you my car next week” is not an enforceable contract.
Capacity means both signers are legally able to agree. Minors and people with severe mental impairment generally lack full capacity, which affects enforceability later. Legality means the purpose cannot violate the law. No matter how carefully drafted, an agreement to commit a crime goes nowhere in court.
Contracts Classified by How They Form
Express Contracts
Express contracts spell out the terms directly, in writing or by spoken word. A signed lease, an employment agreement, a purchase order. The Restatement (Second) of Contracts notes that a promise may be stated in words, oral or written, or inferred partly from conduct, so an agreement can mix express terms with inferred ones.2Open Casebook. Restatement Second of Contracts 4 – How a Promise May Be Made
Implied-in-Fact Contracts
Some contracts form through conduct rather than conversation. When you sit down at a restaurant, read the menu, and place an order, nobody recites prices or asks for a signature. Your behavior creates an implied-in-fact contract: the restaurant has to serve the food, and you have to pay the menu price. Courts look at the parties’ conduct, the setting, and whether a reasonable person would understand that both sides expected an exchange of value.
Quasi-Contracts
A quasi-contract is not really a contract. Courts impose one after the fact to stop someone from receiving a windfall at another person’s expense. The usual example is emergency medical care: a doctor treats an unconscious patient who never agreed to anything. The patient received a real benefit, the doctor had no chance to negotiate, and a court will create a quasi-contractual duty to pay the reasonable value of the services. The parties never reached an agreement at all; the obligation exists because fairness demands it.
Bilateral and Unilateral Contracts
Bilateral Contracts
Most contracts are bilateral. Each side makes a promise in exchange for the other side’s promise. You agree to pay $30,000 for a car; the dealer agrees to deliver it next Tuesday. Both parties are bound the moment they exchange promises, before anyone performs. Employment agreements, service contracts, and real estate deals almost always work this way.
Unilateral Contracts
A unilateral contract trades a promise for a completed act, not a return promise. The offer stays open until someone finishes the requested performance. Reward offers are the classic case. If you post a flyer offering $500 for the return of your lost dog, no one is obligated to look. The moment someone finds and returns the dog, your duty to pay kicks in. Until the act is complete, there is no contract.
What happens if someone starts performing but has not finished? Under the Restatement (Second) of Contracts, beginning the invited performance creates an option contract that stops the offeror from revoking while the offeree is still working.3Open Casebook. Restatement Second Contracts 87 – Option Contract The duty to pay still depends on the offeree actually finishing, but the offeree gets a reasonable chance to complete the job.
Option Contracts
An option contract holds an offer open for a set period, keeping the offeror from revoking it or selling to someone else in the meantime. Real estate deals use these often, with a buyer paying earnest money for the right to purchase within a specified window. The Restatement treats an option as binding if it is in writing with a recital of consideration and proposes a fair exchange within a reasonable time, or if the offeree reasonably relied on the offer in a substantial way before accepting.3Open Casebook. Restatement Second Contracts 87 – Option Contract If the buyer walks away, the deposit is usually lost, but the seller cannot yank the deal while the option period is running.
Valid, Void, Voidable, and Unenforceable Contracts
Valid Contracts
A valid contract satisfies every legal requirement and carries full enforceability. If one side breaches, the other can sue for damages or sometimes ask a court to order performance. This is the baseline. The next three categories describe what has gone wrong.
Void Contracts
A void contract is treated as if it never existed. Illegality is the usual reason. An agreement to sell stolen goods, fix prices, or commit fraud is void from the start. Neither party can enforce it, and neither can recover damages for the other’s failure to perform.
Voidable Contracts
Voidable contracts look valid but give one party the power to walk away. The protection typically applies when the signer lacked full capacity: minors and people suffering from mental impairment or severe intoxication. The protected party chooses whether to cancel or go forward. Cancellation makes the contract void; affirmation makes it fully enforceable. The other party gets no such option. Only the person the law protects can pull the plug.
Unenforceable Contracts
An unenforceable contract has all the right ingredients but hits a procedural wall. The most common barrier is the Statute of Frauds, which requires certain categories of agreement to be in writing. An oral agreement for the sale of goods worth $500 or more generally cannot be enforced in court, even if both parties agree the deal happened.4Legal Information Institute. Uniform Commercial Code 2-201 – Formal Requirements Statute of Frauds Other agreements that generally must be in writing include deals that cannot be performed within one year, promises to pay someone else’s debt, and real estate transfers. The underlying bargain may be perfectly fair, but without the documentation, a judge will not enforce it.
Executed and Executory Contracts
Executed Contracts
An executed contract is one both sides have fully performed. Every obligation has been met. A simple retail purchase becomes executed the moment you hand over payment and leave with the merchandise.
Executory Contracts
An executory contract has outstanding obligations on one or both sides. A 12-month lease is executory from the day you move in. You still owe future rent, and the landlord still owes you habitable premises. A construction contract stays executory until the builder finishes the work and the owner makes the final payment.
Substantial Performance
Performance in the real world is rarely perfect. A contractor might finish a house but install the wrong shade of bathroom tile. Under older common law, any deviation meant breach and no recovery. Modern law takes a more practical view through the doctrine of substantial performance: when a party has completed nearly all of what the contract required, the other side still owes payment but can deduct the cost of fixing the minor deficiencies. The test is materiality. A missing bathroom tile probably is not material. A missing roof certainly is.
Adhesion Contracts
An adhesion contract, sometimes called a standard-form contract, is written entirely by the party with superior bargaining power and offered on a take-it-or-leave-it basis. Cell phone agreements, software licenses, insurance policies, gym memberships. You cannot negotiate the terms. You sign or you walk.
Courts generally enforce these contracts because modern commerce depends on them, but they look at them more closely than negotiated deals, especially when a clause heavily favors the drafter. If a court finds a provision unconscionable, meaning so one-sided that enforcement would shock the conscience, the court can refuse to enforce that clause, enforce the rest of the contract without it, or limit the clause’s application to avoid an unfair result.5Legal Information Institute. Uniform Commercial Code 2-302 – Unconscionable Contract or Clause Arbitration clauses buried in consumer contracts are a common target.
Installment, Requirements, and Output Contracts
Some contracts split performance into pieces or leave the quantity flexible.
An installment contract spreads performance across separate deliveries or payments rather than a single exchange. A supplier shipping 1,000 widgets a month for a year, or a buyer making 24 monthly payments on equipment. Under the Uniform Commercial Code, a deal qualifies as an installment contract if it requires or authorizes delivery of goods in separate lots to be separately accepted, even if the agreement calls each delivery a separate contract. The breach rules are more forgiving than for single-delivery deals. One bad batch does not automatically let the buyer cancel the whole arrangement, though a pattern of nonconformity that substantially impairs the value of the entire contract can.
Requirements and output contracts leave the quantity term open by tying it to one party’s needs or production. A requirements contract obligates a buyer to purchase all of a particular good from a single seller, with the amount set by what the buyer actually needs. An output contract flips that: the buyer takes everything the seller produces. Both are enforceable under the UCC, with a safeguard built in. Neither side can demand or deliver a quantity unreasonably out of proportion to any stated estimate or to prior patterns of output or requirements.6Legal Information Institute. Uniform Commercial Code 2-306 – Output, Requirements and Exclusive Dealings A buyer cannot suddenly triple orders to exploit a favorable price, and a seller cannot slash production to redirect goods elsewhere.
Third-Party Beneficiary Contracts
Most contracts bind only the people who signed. Sometimes, though, an agreement is designed specifically to benefit someone outside the deal. Life insurance is a clean example. You pay premiums to the insurer, and the insurer promises to pay your spouse when you die. Your spouse never signed anything, but the whole point of the contract is to benefit them.
The law separates intended beneficiaries from incidental ones. An intended beneficiary — like the spouse in that example, or a creditor the contract was set up to pay — can sue to enforce the agreement. An incidental beneficiary, someone who happens to benefit from a contract between other people, has no enforcement rights. If a city hires a contractor to repave your street and your property value rises, that is a nice side effect, but you cannot sue the contractor if the work never happens. Intended beneficiaries gain enforceable rights once they learn of the contract and either assent to it, rely on it, or file suit.
Written Contracts and the Parol Evidence Rule
When parties put a deal in writing and intend it as their final word, earlier conversations and side deals that contradict the written terms get shut out. The UCC puts it directly for the sale of goods: terms in a final written agreement cannot be contradicted by evidence of any prior agreement or a contemporaneous oral agreement.7Legal Information Institute. Uniform Commercial Code 2-202 – Final Written Expression Parol or Extrinsic Evidence The written terms can still be explained by trade usage or course of dealing, and consistent additional terms can come in unless the court finds the writing was meant as the complete and exclusive statement of the deal.
Many commercial contracts include a merger clause, sometimes called an integration clause, stating that the written document is the entire agreement and supersedes all prior negotiations. If your contract has one, do not count on enforcing a verbal promise the other side made during negotiations but left out of the signed document. Whatever did not make it into the writing is, for practical purposes, gone.
When You Can Cancel and How Long You Have to Sue
Two practical rules sit alongside the categories above. Federal law gives you a narrow right to cancel certain sales after signing. Under the FTC’s Cooling-Off Rule, you have three business days to cancel a sale made at your home, your workplace, or a temporary seller location such as a hotel room or convention center. The rule does not cover online, mail, or telephone purchases, and it does not apply to sales made at the seller’s permanent business location. Real estate transactions, insurance policies, securities, and motor vehicle sales at temporary locations where the seller also keeps a permanent store are all excluded, and the rule only kicks in above minimum dollar thresholds of $25 for home sales and $130 for temporary-location purchases.8Federal Trade Commission. Buyers Remorse The FTCs Cooling-Off Rule May Help
Every contract dispute also has a deadline. Statutes of limitation for breach of a written contract range from 3 to 15 years depending on the jurisdiction, with most states falling between 4 and 6 years. Oral contracts typically have shorter periods. The clock usually starts when the breach occurs, not when you discover it, so a delay in recognizing the problem can be expensive. If you think someone has broken a contract with you, checking your state’s deadline is one of the first things worth doing.