Most contracts are bilateral in the sense that X and Y each promise to give or do something. But with unilateral contracts it is only the offeror that makes a promise. For example, if X says, ‘I will pay a £100 reward to anyone who finds and returns my gold ring to me’, he has made a promise that is not reciprocated by anyone, since nobody is bound to find and return his gold ring. Such contracts are sometimes referred to as ‘if’ contracts as in: ‘If you do X, I promise to do Y’. Moreover, whereas a bilateral agreement is a promise in return for a promise, a unilateral agreement is a promise in return for an act (unsupported by any promise to actually do the act).
Due to their different nature unilateral contracts have different rules regarding:
• Unilateral advertisements are likely to be construed as offers rather than invitations to treat.
• Acceptance must be in response to the offer, and with knowledge of it. There is no binding contract if a person just happens to find the missing ring and, since he knows the owner, returns it to him.
• Revocation. A unilateral offer may not be revoked once the promisee has partially performed, or started to perform, the required act. For example, if X makes a unilateral offer to pay £100 to anyone who walks from London to Newcastle, X cannot revoke the offer whenever anyone gets close to Newcastle. (1)
(1) Carlill v Carbolic Smoke Ball Company  1 QB 256, CA.