The main types of contracts are grouped along four lines: whether a court will enforce them, how they were formed, what each side promised, and how much of the deal has actually been performed. The same agreement can carry a label on each axis at once, so a written employment agreement might be valid, express, bilateral, and executory all at the same time. The labels matter because each one changes what you can do if the other side fails to follow through.
Contracts Grouped by Enforceability
Not every agreement a court will look at is one a court will enforce. The Restatement (Second) of Contracts defines a contract as a promise or set of promises for which the law provides a remedy or recognizes performance as a duty.1Open Casebook. Restatement Second of Contracts 1-2, 178 Four labels cover the spectrum.
A valid contract meets every legal requirement: a genuine offer, acceptance of that offer, consideration (something of value exchanged), and parties who have the legal capacity to agree. With all four elements present and a lawful subject, either side can go to court for damages or to force performance if the other fails to follow through.2Legal Information Institute. Contract
A void contract has no legal effect from the start. Agreements to do something illegal or physically impossible fall here. Because these arrangements are treated as if they never existed, neither side can enforce the terms or claim damages for nonperformance.
A voidable contract looks valid on the surface but gives one party the power to walk away. Common grounds are age (contracts with minors are voidable at the minor’s option), duress, fraud, and misrepresentation. The Restatement (Second) of Contracts states that a contract induced by an improper threat leaving you no reasonable alternative is voidable by you as the victim.3Open Casebook. Restatement Second of Contracts 175 If the protected party later ratifies the deal, it becomes fully binding. If they disaffirm it, the contract unravels.
An unenforceable contract is technically valid but blocked by a procedural barrier. The two usual barriers are the Statute of Frauds (requiring certain contracts to be in writing) and expiration of the statute of limitations. The agreement still exists, and both sides may have genuinely intended to follow through, but a court will not grant a remedy once the deadline or writing requirement has been missed. For goods, the Uniform Commercial Code sets its own threshold: a contract for the sale of goods priced at $500 or more is not enforceable unless there is a signed writing sufficient to show that an agreement was made.4Legal Information Institute. UCC 2-201 – Formal Requirements; Statute of Frauds
Contracts Grouped by How They Are Formed
Some agreements are spelled out word by word. Others arise from what people do rather than what they say, and in a few cases a court imposes contract-like duties where no one agreed to anything at all.
Express Contracts
An express contract is what most people picture. The parties state their terms in words, spoken or written. A signed lease, an employment agreement, or a purchase order with listed prices and delivery dates all qualify. The obligations are communicated directly through language rather than inferred from behavior.
Implied-in-Fact Contracts
An implied-in-fact contract forms through conduct. Sit down at a restaurant and order food, and you have not signed anything, but your actions create a binding promise to pay the menu price. The restaurant cooking and serving the food creates a reciprocal obligation to deliver what you ordered. Courts look at the circumstances and ask whether a reasonable person would conclude that both sides intended to be bound.
Quasi-Contracts
A quasi-contract is not really a contract at all. It is a tool courts use to prevent one person from unfairly benefiting at another’s expense. The classic example involves emergency medical care: if you are unconscious and a doctor treats you, no one negotiated any terms, but a court will impose an obligation to pay the reasonable value of the care you received. The legal label for this is unjust enrichment. Letting someone receive a substantial benefit without paying anything would produce an unfair result, even though no agreement was ever discussed.
Contracts Grouped by What Each Side Promises
The structure of who promises what shapes when the contract becomes binding and how either side gets out.
Bilateral Contracts
A bilateral contract is a promise exchanged for a promise. Both sides commit to doing something. In a standard sales agreement, the seller promises to deliver a product and the buyer promises to pay a specific price. Both parties are bound the moment they exchange promises. The vast majority of everyday contracts are bilateral: employment agreements, leases, service contracts, and purchase orders.
Unilateral Contracts
A unilateral contract involves a promise from one side that can only be accepted by the other side’s actual performance. The textbook example is a reward offer: someone posts a notice promising $500 to whoever finds and returns their lost dog.5Legal Information Institute. Unilateral Contract Nobody is obligated to look for the dog. No contract exists until someone actually returns it. Insurance policies share this structure: the insurer promises to pay if a covered loss occurs, and the policyholder’s payment of premiums is the consideration that keeps the promise alive.
Option Contracts
An option contract gives one party the right, but not the obligation, to enter into a future agreement on specified terms. The person granting the option receives consideration (usually a payment) in exchange for keeping the offer open for a set period. Real estate deals use option contracts frequently: a developer might pay a landowner $10,000 for a 90-day exclusive right to purchase a parcel at a set price.
Under the UCC, merchants get a special rule. A written, signed offer by a merchant to buy or sell goods that promises to stay open is binding without separate consideration for up to three months.6Legal Information Institute. Option Contract Outside the merchant context, most option contracts require actual consideration to be enforceable.
Contracts Grouped by Performance Status
Performance status describes where things stand in the life of the contract. An executed contract is one where both sides have fully performed. At a real estate closing, once the deed transfers and the buyer pays the purchase price, the contract is executed. No one owes anything further.
An executory contract is one where obligations remain unfulfilled on one or both sides. A year-long apartment lease is executory for its entire duration because the tenant owes monthly rent and the landlord owes continued access to the property.7United States Bankruptcy Court. What Is an Executory Contract and Why Must I Assume or Reject It? A contract stays executory until the last duty on both sides is discharged. This classification matters most in bankruptcy, where executory contracts must be specifically assumed or rejected.
Electronic and Digital Contracts
Federal law has made clear that a contract cannot be denied legal effect solely because it was formed electronically. Under the Electronic Signatures in Global and National Commerce Act (E-SIGN), any signature, contract, or record relating to interstate commerce is legally valid even if it exists only in electronic form.8Office of the Law Revision Counsel. 15 USC 7001 – General Rule of Validity The Uniform Electronic Transactions Act, adopted in most states, reinforces this by providing that electronic records and signatures satisfy any law requiring a writing or a signature.
In practice, most online agreements take one of two forms, and the distinction matters in court.
A clickwrap agreement requires you to take an affirmative action, like checking a box or clicking “I Agree,” before proceeding. Courts generally consider these enforceable because the click shows you were aware terms existed and chose to accept them. Software installations, online purchases, and SaaS signups almost universally use this format.
A browsewrap agreement assumes you consented to terms simply by using a website. The terms typically sit behind a hyperlink buried in the page footer, and you are never asked to do anything to signal agreement. Courts view these with far more skepticism. The key question is whether the terms were conspicuous enough that a reasonable person would have noticed them. If the link blends into the background and nothing draws attention to it, a court is unlikely to enforce the terms.
Adhesion Contracts and Unconscionability
An adhesion contract is a standardized agreement drafted entirely by the party with stronger bargaining power and presented on a take-it-or-leave-it basis. Cell phone service agreements, insurance policies, car rental forms, and software licenses all fit this description. You either sign or you do not get the service. There is no realistic opportunity to negotiate individual terms.
Adhesion contracts are not automatically unenforceable, but courts have tools to police the worst abuses. Under the UCC, if a court finds that a contract or any clause was unconscionable at the time it was made, the court can refuse to enforce the contract entirely, strike the offending clause, or limit the clause to avoid an unconscionable result.9Open Casebook. UCC 2-302 – Unconscionable Contract or Clause
Courts evaluating unconscionability look at two dimensions. Procedural unconscionability focuses on the negotiation process: was one side ambushed by hidden terms they had no realistic chance of reading or understanding? Substantive unconscionability focuses on the terms themselves: are they so one-sided that enforcing them would shock the conscience? A contract buried in fine print that quietly strips you of your right to sue and caps the company’s liability at zero is the kind of provision courts are willing to invalidate. Finding unconscionability on both dimensions strengthens the case, though some courts will act when one dimension is especially severe.
What Every Type Still Needs: Consideration and Capacity
No matter how a contract is categorized, it generally needs consideration to be enforceable. Consideration means each side must give something and receive something in return. The exchange must be bargained for: the promise and the performance need to be connected as part of the same deal.10Open Casebook. Restatement Second Contracts 71 – Consideration
Consideration does not have to be money. It can be a service, a transfer of property, a promise to do something, or even a promise to refrain from doing something you have a legal right to do. Courts generally do not require the exchange to be equal or fair. What fails as consideration: a promise to give a gift with nothing expected in return, a promise to do something you are already legally obligated to do, and payment for work completed before the promise was made. Past favors do not count because consideration must be part of the current agreement, not a reward for something already done.
Capacity is the other floor. The age of majority is 18 in most states. Contracts entered into by minors are voidable at the minor’s option, meaning the minor can walk away but the adult cannot. A minor who wants to disaffirm a contract generally must do so within a reasonable time after turning 18. The major exception: contracts for necessities like food, shelter, clothing, and basic medical care, where courts will hold minors liable for the reasonable value of what they actually received.
Mental capacity follows a similar logic. If a person cannot understand the nature and consequences of the transaction, the contract is voidable. This can result from mental illness, cognitive decline, or severe intoxication. For intoxication, the other party must have known or should have known about the impairment. Simply having a drink before signing a contract does not give you an escape hatch.