A contract under which a surety (the guarantor) promises to be responsible for the performance of an obligation owed by a principal obligor to a third party if the principal obligor fails to perform the obligation.

The guarantor's liability under a guarantee is a secondary liability, as the guarantor is not liable unless and until the principal obligor fails to perform its own obligation to the third party (the primary liability).

There are two types of guarantee, which are generally referred to as see-to-it guarantees or pure guarantees and conditional payment guarantees.

A guarantee is not enforceable unless it, or a memorandum or note of it, is in writing and signed by the guarantor or at the guarantor's direction (section 4, Statute of Frauds (of 1677)).

Guarantees should be distinguished from indemnities, on-demand bonds (also sometimes known as demand guarantees) and standby letters of credit.