The basic principles of contract law come down to six ingredients that every enforceable agreement needs: an offer, an acceptance, consideration, capacity, a lawful purpose, and an intent to be legally bound. When all six are present, you have a contract a court will enforce. When any one is missing, or when something went wrong in how the agreement was made, the deal can collapse. The sections below walk through each element, the situations that require a written document, the defenses that can undo a contract, and what happens when one side fails to perform.
Offer and Acceptance
A contract starts with one party making an offer on specific terms and the other accepting those terms. The offer has to be concrete enough that both sides know what they’re committing to. Telling a neighbor “I might sell my car sometime” isn’t an offer. Saying “I’ll sell you my car for $12,000, with delivery by Friday” is. The Restatement (Second) of Contracts describes mutual assent as ordinarily taking the form of an offer followed by an acceptance.1H2O. Restatement (Second) of Contracts 22 – Mode of Assent: Offer and Acceptance
Acceptance must match the offer. This is called the mirror image rule. Change a single material term while responding, and you’ve made a counteroffer instead of an acceptance. The original offer is gone. If the seller asks $12,000 and the buyer answers “I’ll take it for $10,000,” no contract exists, and the seller is free to walk away.
Timing follows the mailbox rule. Acceptance is effective when it’s properly sent, not when it’s received. Mail a signed acceptance on Tuesday and the contract forms on Tuesday, even if the envelope sits unopened until Friday. The exception is when the offer itself specifies a method or a date for receipt. Then the offeror’s instructions control.
Consideration
An agreement without consideration is just a promise, and bare promises aren’t enforceable. Consideration means each side gives up something of value or takes on a new obligation as part of the exchange. Under the Restatement, a performance or return promise counts as consideration when both parties bargain for it — the promisor seeks it in exchange for the promise, and the promisee gives it in exchange for that promise.2H2O. Restatement (Second) of Contracts 71 – Requirement of Exchange; Types of Exchange
The exchange doesn’t have to be money. You can trade services, hand over property, or give up a legal right you already hold. Agreeing not to sue in return for a settlement payment is a classic example of that last category. What courts care about is that something is being exchanged, not that the exchange is objectively fair. A $100 painting that later proves to be worth $50,000 doesn’t invalidate the sale. The one time a court looks harder at the balance is when the lopsidedness itself suggests fraud, coercion, or some other defect in how the deal was made.
When a Promise Without Consideration Can Still Be Enforced
Promissory estoppel is the main exception. If someone makes a clear promise, reasonably expects the other person to rely on it, and that person does rely on it to their detriment, a court can enforce the promise even without traditional consideration.3H2O. Restatement (Second) of Contracts 90 – Promise Reasonably Inducing Action or Forbearance
Picture an employer who promises a job to a candidate who then quits her existing position, moves across the country, and signs a lease. If the employer rescinds before the start date, the candidate has suffered real losses from reasonable reliance on a clear promise. A court can step in under promissory estoppel. The remedy usually covers the actual losses from the reliance, not the full value the promised deal would have delivered.
Capacity to Contract
Both parties need the legal ability to understand and commit to the deal. Two groups come up most often in capacity disputes.
Minors
A person under eighteen can enter a contract, but those contracts are voidable at the minor’s option. The minor can back out through a process called disaffirmance. The adult on the other side has no equivalent escape. The law treats minors as unable to fully appreciate long-term consequences, so the protection runs one direction. When a minor disaffirms, they generally must return whatever they received. For a physical item like a car, that means giving it back; for digital purchases or consumed services, the return rules depend on the state.
There are exceptions. Most states allow minors to enter binding contracts for necessities like food, clothing, shelter, and medical care. Some permit minors above a certain age to contract for specific products like insurance. Once a minor turns eighteen, they can ratify any contract they entered earlier, either explicitly or by continuing to accept its benefits without objection.
Mental Capacity
Adults can also lack capacity if a mental illness, cognitive disability, or severe intoxication prevents them from understanding what they’re agreeing to. The question is whether the person could comprehend the nature and consequences of the transaction at the time of signing. A contract made during a temporary episode of incapacity is voidable, but if the other party had no reason to know about the condition and the terms were fair, a court will weigh whether voiding the agreement would create its own injustice.
Lawful Purpose
A contract must call for something legal. If the agreement requires either side to do something illegal — sell controlled substances, commit fraud, run an unlicensed gambling operation — no court will enforce it. The contract is void from the start. Even if both parties willingly performed, neither can sue the other for breach.
The rule reaches beyond outright crime. Agreements that violate public policy can also fail. A clause requiring professional malpractice would be struck down. Unreasonable restrictions on competition get similar treatment, though the enforceability of non-compete agreements varies significantly by state and continues to shift.
Intent to Create Legal Relations
Even with every other element in place, the parties must actually mean to be legally bound. In commercial settings this is almost always presumed. When two businesses sign a supply agreement, no one has to prove they intended enforceability.
Social and family arrangements work the opposite way. A friend’s promise to help you move, or a relative’s pledge to lend you money, generally lacks enforceable legal intent. Courts apply an objective standard: would a reasonable outside observer, looking at the language, the context, and the behavior of the parties, believe this was meant to be a binding deal? Written terms, formal language, and the exchange of money point toward legal intent. Casual conversation and family dynamics point away from it.
When a Contract Must Be in Writing
Oral contracts are enforceable in many situations, but a doctrine called the statute of frauds requires certain categories of agreements to be in writing. The specifics vary by state. The most common categories include:
- Sales of goods worth $500 or more. Under the Uniform Commercial Code, a contract for the sale of goods at or above this threshold needs a written record signed by the party you’re trying to hold to the deal.4Legal Information Institute. UCC 2-201 – Formal Requirements; Statute of Frauds
- Real estate transactions. Contracts to buy, sell, or transfer an interest in land almost always must be in writing. Short-term leases, usually under one year, are a common exception.
- Agreements that cannot be performed within one year. If the contract by its terms cannot possibly be completed within a year from formation, it needs to be in writing. The key word is “cannot.” If there’s any plausible way the work could finish within twelve months, the writing requirement doesn’t apply, even if performance actually takes longer.
- Promises to pay someone else’s debt. Guaranteeing a friend’s loan is a promise of suretyship and must be in writing to be enforceable against you.
A written contract doesn’t need to be a formal document full of legal jargon. A signed letter, an email exchange, or even a text message chain can satisfy the writing requirement as long as it identifies the parties, describes the subject matter, and is signed or otherwise authenticated by the person being held to it.
Electronic Signatures
Federal law treats electronic signatures and electronic records the same as their ink-and-paper equivalents. Under the E-SIGN Act, a contract cannot be denied legal effect simply because it was formed using electronic signatures or exists only as a digital record.5Office of the Law Revision Counsel. 15 USC 7001 – General Rule of Validity The one catch is that the electronic record must be kept in a form all parties can access and reproduce later. Signatures captured through platforms like DocuSign and Adobe Sign satisfy this requirement in virtually all commercial and consumer transactions.
Defenses That Can Void a Valid-Looking Contract
A contract that checks every formation box can still be challenged if something went wrong in how it was made. These defenses don’t argue that no contract exists. They argue the contract is defective and shouldn’t be enforced.
Fraud and Misrepresentation
If one party lied about something important to get the other to sign, the contract is voidable. The deceived party must show that the other side made a false statement about a material fact, intended to induce agreement, and that the deceived party reasonably relied on the lie. Selling a car while hiding known engine damage is the textbook example. The seller’s lie about the car’s condition induced the buyer to pay more than the car was worth, and the buyer had no easy way to find out the truth before signing.
Duress and Undue Influence
A contract signed under threat is voidable. Duress can be physical (“sign this or I’ll hurt you”) but more often it’s economic — one party threatens to breach an existing obligation or withhold something critical unless the other agrees to unfavorable new terms. The victim must show they had no reasonable alternative to signing. Undue influence is a quieter cousin of duress. It arises when someone in a position of trust or authority (a caretaker, a financial advisor, a family member with control over an elderly relative) exploits that relationship to push through a one-sided deal.
Unconscionability
Courts can refuse to enforce a contract, or strike individual clauses, when the terms are so one-sided they “shock the conscience.” The UCC gives courts explicit authority to do this.6H2O. UCC 2-302 – Unconscionable Contract or Clause Courts look at two dimensions. Procedural unconscionability focuses on how the deal was made: was there a huge gap in bargaining power, hidden fine print, or no real chance to negotiate? Substantive unconscionability focuses on the terms themselves: is the price wildly above market, does one party bear all the risk, are the penalty provisions absurdly harsh? A contract is most vulnerable when both are present.
Mutual Mistake
When both parties are wrong about a basic fact underlying the deal, the agreement can be voided. The classic example is a sale of a painting both parties believe to be a reproduction that turns out to be an original worth a fortune. The mistake must concern a fundamental assumption of the deal and must materially affect the exchange. A mistake about a minor detail, or a risk one party explicitly took on, won’t qualify.
Good Faith and the Parol Evidence Rule
Every contract carries an implied duty of good faith and fair dealing in its performance and enforcement.7H2O. Restatement (Second) of Contracts 205 – Duty of Good Faith and Fair Dealing You don’t have to write this duty into the agreement; it’s built in. In practice, it means neither side can use technicalities or bad-faith tactics to undermine the other’s right to receive the benefits they bargained for. An employer who fires an at-will employee the day before a large commission vests, solely to avoid paying it, could face a good-faith claim even though the employment contract technically allowed termination at any time.
Once a written contract is signed and intended as the final word on the deal, the parol evidence rule limits what outside information you can bring in later. You generally can’t introduce earlier conversations, prior drafts, or side agreements to contradict the signed document. If the contract says the price is $50,000, you can’t testify that you and the seller verbally agreed to $40,000 before signing. The rule has limits. Evidence of fraud, duress, or mutual mistake can come in because those defenses attack the validity of the contract itself. Courts also allow evidence of trade customs, prior dealings between the parties, and consistent additional terms that don’t contradict the writing, particularly when the written document doesn’t cover every aspect of the relationship.
What Happens When Someone Breaches
A breach occurs when one party fails to perform their obligations. Not all breaches are treated equally, and the severity determines what the injured party can do.
Material vs. Minor Breach
A material breach goes to the heart of the deal and deprives the other party of the main benefit they expected. Hire a contractor to build a garage and they never show up — that’s material. You can stop your own performance, terminate the contract, and pursue damages. A minor breach is a less serious failure that doesn’t defeat the main purpose of the agreement. If the contractor builds the garage but uses a slightly different shade of paint than specified, the contract stays in force. You still owe payment, but you can seek damages for the cost of correcting the paint. Getting this distinction right matters because treating a minor breach as material, by walking away from the contract, can itself become a breach.
Types of Damages
The default remedy for breach is money damages. There are three main categories:
- Expectation damages. These put you in the financial position you would have been in if the contract had been performed. The calculation starts with the value you lost from the other side’s failure, adds any incidental or consequential losses the breach caused, and subtracts any costs you avoided by not having to finish your own performance.8H2O. Restatement (Second) of Contracts 347 – Measure of Damages in General
- Reliance damages. When the expected profit from the deal is too speculative to calculate, courts can instead restore you to where you were before the contract was formed. These cover expenses you incurred in reasonable reliance on the other party’s promise.
- Restitution. When you’ve already conferred a benefit on the breaching party — delivered materials, performed work, made a payment — restitution returns the value of what you gave them. This stops the breaching party from profiting at your expense.
Specific Performance
Sometimes money isn’t enough. When the subject of the contract is unique (a specific piece of real estate, a one-of-a-kind artwork, a rare collectible), a court can order the breaching party to actually perform rather than pay damages. This remedy is most common in real estate transactions because every parcel of land is legally considered unique. Courts are reluctant to order specific performance for personal service contracts, where forcing someone to work against their will raises serious practical and constitutional concerns.
Liquidated Damages
Parties can agree in advance on a fixed amount of damages owed if one side breaches. These clauses are enforceable when the agreed amount is a reasonable estimate of the probable loss and the actual damages would be hard to calculate at the time of contracting. If the number is so high that it functions as a punishment rather than compensation, a court will strike it down as an unenforceable penalty. Construction contracts frequently use liquidated damages clauses to set a per-day charge for project delays, since the downstream costs of late completion are genuinely hard to measure in advance.
Duty to Mitigate
The injured party can’t sit back and let losses pile up. Contract law requires you to take reasonable steps to minimize your damages after a breach. If a tenant breaks a lease early, the landlord can’t leave the unit vacant for twelve months and then sue for all the remaining rent. The landlord has to make a reasonable effort to find a new tenant. The standard isn’t perfection, and nobody has to accept a clearly inferior substitute, but a court will reduce the damages award by whatever amount you could have reasonably avoided.
Deadlines for Filing a Breach Claim
Every breach of contract claim has a filing deadline. Miss it and you lose the right to sue, no matter how strong your case. These statutes of limitation vary by state, with most falling between three and ten years. Written contracts generally carry longer limitation periods than oral agreements; in some states, written contract claims can be filed up to ten or even fifteen years out while oral contract claims must be filed within three to five. The clock typically starts running on the date the breach occurs, not the date you discover it. Waiting to “see if things work out” can be a costly mistake.