Generally, contracts do not have to be written down to be legally binding, although sometimes it can be harder to prove exactly what was agreed to in an oral contract.
Some contracts, however, are legally required to be in writing, including:
While a contract is not required to be in writing for it be enforceable, it may be advisable to have it in writing regardless. This may prevent or limit a dispute at a later time.
For example, if a contract is of special importance, involves a substantial sum of money, or if there is a possibility of a dispute about it in the future, it is wise to have a written agreement.
Some agreements will not be legally enforceable, even if written down and signed. These include agreements where there was no consideration, or agreements to do things that are illegal – see Illegal Contracts.