The short answer

People make agreements all day — a handshake, a text message, a tick in an "I agree" box. But not every agreement is a contract. A contract is a promise (or set of promises) that a court will actually enforce. If the right ingredients are missing, what you have is just an understanding, and the other side may be able to walk away with no consequences.

The good news is that the core ingredients are remarkably similar across most of the world. The details and the names differ from country to country, but almost every legal system asks the same basic questions before it will treat an agreement as binding.

1. A clear offer and a clear acceptance

Every contract starts with one side proposing specific terms (the offer) and the other side agreeing to those exact terms (the acceptance). "I'll sell you my car for 5,000" followed by "Yes, I'll take it at that price" is a clean offer and acceptance.

Where people get caught out is the difference between a real offer and an invitation to negotiate. A price tag in a shop, an advert, or a listing is usually treated as an invitation to make an offer, not the offer itself. And if you reply with different terms ("I'll pay 4,500 instead"), you have made a counter-offer — the original offer is gone until the other side accepts your new one.

2. Something of value must change hands

A contract is a two-way street. Each side has to give up something — money, goods, services, or a promise to do (or not do) something. In common-law countries this is called consideration; many civil-law systems use the related idea of cause or a lawful reason for the obligation.

The practical point is the same everywhere: a bare promise to give someone a gift, with nothing in return, is usually not an enforceable contract. If you promise to pay a friend 100 for nothing, they generally cannot sue you when you change your mind.

3. Both sides must intend to be legally bound

Courts only enforce agreements the parties meant to be legally serious. Commercial and business deals are assumed to be legally binding. Casual social arrangements — agreeing to meet a friend for dinner, or a family promise about pocket money — usually are not, unless something shows the parties really did intend legal consequences.

A useful test: if this fell apart, would a reasonable person expect a court to get involved? For a supplier contract, yes. For a promise to give someone a lift to the airport, almost certainly not.

4. Everyone must have the capacity to agree

The people signing must be legally able to enter a contract. Most systems limit or protect certain groups — for example minors (people under the age of majority), and people who lack mental capacity to understand what they are agreeing to. Contracts with these parties may be void, or voidable at their option.

For businesses, capacity also means the person signing actually has authority to bind the company. Always check that the individual on the other side can genuinely commit their organisation.

5. The purpose has to be legal

A court will not enforce an agreement to do something illegal or against public policy. An arrangement to commit a crime, to defraud someone, or to break a clear legal prohibition is unenforceable no matter how carefully it is written. If the object of the deal is unlawful, the contract falls away with it.

6. The terms must be certain enough

Finally, the agreement has to be definite enough to enforce. If the key terms — price, quantity, timing, exactly what each side must do — are too vague or left "to be agreed later", a court may find there is no contract at all because it cannot tell what the parties actually committed to. Precision protects you.

Does a contract have to be in writing?

Often, no. In most places a spoken agreement can be perfectly binding if the ingredients above are present. But there are important exceptions where the law requires writing — commonly for things like the sale of land or real estate, certain guarantees, some long-term arrangements, and consumer credit. These requirements vary significantly by country and even by region.

Even when writing is not legally required, it is almost always the smarter choice. A written contract is not just a formality — it is your evidence of what was actually agreed if a dispute ever arises.

Are verbal, email and text agreements binding?

They can be. A deal struck by phone, email or messaging app can meet every requirement of a valid contract. The problem is rarely validity — it is proof. Months later, memories differ and screenshots go missing. If a matter is worth anything to you, get the key terms confirmed in a single clear document that both sides accept.

What about clicking "I agree" and e-signatures?

Online terms and conditions and electronic signatures are widely recognised as capable of forming binding contracts, and most legal systems now have specific laws confirming this. Clicking "I accept" after being given a genuine chance to read the terms will usually count as acceptance. The main open questions are whether the terms were fairly presented and whether any of them are so unusual or one-sided that consumer-protection rules cut them down.

Red flags to check before you sign

  • Blanks and "TBD" terms. Never sign with key details left empty or to be filled in later.
  • Automatic renewals and lock-ins. Look for clauses that renew the contract or make it hard to leave.
  • One-sided cancellation or penalty terms. Check what happens if either side wants out, and whether the penalties are balanced.
  • "Entire agreement" clauses. These often mean spoken promises made during negotiation do not count — so get them into the document.
  • Which law and which courts apply. Cross-border deals should say whose law governs and where disputes are decided.
  • Anything you do not understand. If a clause is unclear, ask before you sign, not after.

When to get professional advice

The principles above will help you spot whether an agreement is likely to be binding and what to watch for. But contract law genuinely differs from one jurisdiction to the next — on writing requirements, consumer protections, capacity rules, and how courts read unclear terms. For anything high-value, long-term, or important to you, have a qualified lawyer in your own country review it before you commit. A short review up front is almost always cheaper than a dispute later.