A legally binding contract has four requirements: an offer, acceptance of that offer, consideration exchanged between the parties, and parties with legal capacity entering an agreement for a lawful purpose. Miss any one of them and the deal falls apart, no matter how formal the paperwork looks or how firmly everyone shook hands.
Offer
A contract starts when one party proposes a deal on clear, specific terms. The person making the proposal is the offeror, and the person receiving it is the offeree. The offer has to be definite enough that both sides know what they are agreeing to: the subject matter, the price or exchange, and any key conditions. Vague suggestions and expressions of interest do not count.
An offer does not stay open forever. Several things can end it before anyone accepts:
- Revocation. The offeror can pull the offer back any time before acceptance, but the revocation only takes effect once the offeree actually learns about it.
- Rejection. If the offeree says no, the offer dies. A counteroffer counts as a rejection because it proposes different terms, which wipes out the original offer.
- Lapse of time. If the offer states a deadline, it expires when the deadline passes. If no deadline is stated, it expires after a reasonable time, which depends on the circumstances.
- Death or incapacity. If either party dies or becomes mentally incapacitated before acceptance, the offer generally lapses.
Acceptance
Acceptance is the offeree’s clear agreement to the exact terms of the offer. It has to show an intent to be bound and has to be communicated back to the offeror. Courts judge acceptance objectively: what matters is what the offeree expressed outwardly, not what they privately thought.1Legal Information Institute. Acceptance It can happen through words, written or spoken, or through conduct that clearly shows agreement, such as beginning the work the offer described.
Under common law, acceptance has to match the offer exactly. This is the mirror image rule. Change one term and it is not acceptance anymore; it becomes a counteroffer, which rejects the original and proposes new terms in its place.2Legal Information Institute. Counteroffer The original offeror can accept or reject that counteroffer, but the original offer is gone.
Sales of goods work differently. Under the Uniform Commercial Code, which governs commercial transactions in every state, an acceptance can create a binding contract even when it includes additional or different terms compared to the offer.3Legal Information Institute. Mirror Image Rule The looser standard reflects how business deals actually work: purchase orders and confirmations rarely line up word for word.
Timing matters when parties are contracting at a distance. The traditional mailbox rule says acceptance takes effect the moment the offeree sends it, whether that is dropping a letter in the mail or hitting send on an email. Everything else, including revocations and rejections, only takes effect on receipt. If the offeror specifies a method of acceptance, such as “reply by fax only,” the offeree has to use that method for the acceptance to be valid.
Consideration
Consideration is what each side gives up to get what the other side is offering. Every enforceable contract needs each party to provide or promise something of value in exchange for what the other party provides. This bargained-for exchange is what separates a contract from a gift.4Legal Information Institute. Consideration
Consideration can take many forms: money, goods, services, a promise to do something, or even a promise to refrain from doing something you have a legal right to do. If a neighbor agrees to stop playing drums after 10 p.m. in exchange for you mowing their lawn every Saturday, both sides have provided consideration.
Courts generally do not care whether the exchange is lopsided. A $1 payment for a car worth $20,000 can still count as valid consideration, though extreme imbalances may signal fraud or coercion. What matters is that both parties are giving or promising something they were not already obligated to provide.
Two things do not qualify. A promise to do something you are already legally required to do fails as consideration under the pre-existing duty rule. If a contractor is already under contract to build your deck for $5,000, a new promise to pay $7,000 for the same work is not supported by fresh consideration because the contractor has not taken on any new obligation.5Legal Information Institute. Pre-Existing Duty Doctrine Past performance does not work either. If someone mows your lawn without being asked and then demands payment, no contract exists, because the work was done before any agreement was reached.
Capacity and Lawful Purpose
Both parties need the legal ability to enter a contract. Capacity means reaching a minimum age and having the mental competence to understand what you are agreeing to.6Legal Information Institute. Capacity
In most states the age of legal capacity is 18. A contract signed by a minor is not automatically invalid, but it is voidable, meaning the minor can choose to honor it or walk away from it.7Legal Information Institute. Voidable The adult on the other side does not get the same escape hatch. Once the minor turns 18, they can affirm the contract and make it fully binding, or disaffirm it and cancel their obligations.
Mental incapacity works the same way. A person who could not understand the nature and consequences of the agreement at the time of signing, whether because of cognitive disability, mental illness, or severe intoxication, may have grounds to void the contract. The key question is whether the person understood what they were signing and what obligations it created.
The agreement also has to involve legal activity. A contract to do something illegal, such as selling stolen goods, rigging a bid, or committing fraud, is void from the start.8Legal Information Institute. Void Void means the contract never had legal force. No court will enforce it, and neither party can sue the other for failing to hold up their end. That is different from voidable: a voidable contract, such as one signed by a minor or obtained through fraud, is valid until the wronged party decides to cancel it, and the wronged party can choose to go forward if that serves their interests.
When a Contract Also Has to Be in Writing
Meeting the four requirements does not always make an agreement enforceable. Under the statute of frauds, certain contracts have to be in writing and signed by the party being held to the deal. Oral versions of these contracts are generally unenforceable even when all four elements are present.
Categories vary somewhat by state, but the most common types that have to be in writing include:
- Real estate transactions. Any contract to buy, sell, or transfer an interest in land.
- Agreements lasting more than one year. If the contract by its terms cannot be fully performed within 12 months from the date it is made, it has to be in writing.
- Promises to pay someone else’s debt. A guarantee that you will cover another person’s obligation to a creditor has to be written.
- Sale of goods above a certain dollar amount. Under the Uniform Commercial Code, contracts for the sale of goods above a specified price threshold, which varies by state, have to be in writing.
This is where people get burned. A verbal agreement to buy a house can satisfy every other legal standard and still be unenforceable because it was never written down.
What Can Still Undo an Otherwise Valid Contract
A contract that checks every box can still be challenged if something went wrong during negotiation. These defenses do not attack the four elements. They argue that the agreement, while technically complete, was tainted by misconduct or unfairness.
Duress
A contract signed under threats or coercion is voidable by the pressured party. Duress means one side used unlawful pressure to destroy the other party’s ability to freely choose whether to agree.9Legal Information Institute. Duress The threat has to be serious enough that a reasonable person would have felt they had no real alternative. Threatening to report someone for a crime unless they sign a business deal is one example.
Undue Influence
Undue influence is subtler. It arises when someone in a position of trust or authority, such as a caregiver, financial advisor, or family member, uses that relationship to pressure a vulnerable person into a one-sided deal.10Legal Information Institute. Undue Influence The person being influenced typically has some vulnerability, whether age, illness, grief, or dependence on the influencer. If proven, the contract becomes voidable at the option of the influenced party.
Fraud and Misrepresentation
If one party lied about a material fact to get the other side to sign, the contract is voidable by the deceived party.11Legal Information Institute. Fraud in the Inducement The misrepresentation has to be about something important to the deal, not sales puffery. Telling a buyer a car “runs like a dream” is vague opinion. Telling them the odometer reading is accurate when you know it has been rolled back is fraud. The deceived party can void the contract or keep it in place and pursue damages.
Unconscionability
A court can refuse to enforce a contract, or strike specific clauses from it, if the terms are so one-sided that they shock the conscience.12Legal Information Institute. UCC 2-302 – Unconscionable Contract or Clause Courts look at both how the contract was formed, including whether one side had no real bargaining power or ability to understand the terms, and at what the contract actually says. Both factors usually need to be present. A lopsided contract between two sophisticated businesses is harder to challenge than the same terms imposed on a consumer who had no opportunity to negotiate.