Determinable describes an estate, interest or right that will automatically end on the occurrence (or non‑occurrence) of a specified event, without any need for further action such as forfeiture or termination notice. In UK and Irish property law, a determinable fee simple or determinable lease continues “until” or “while” a condition is satisfied, and then comes to an end automatically when that condition fails. The term is not usually defined in statute, but is well established in case law and legal commentary across England and Wales, Scotland, Northern Ireland and Ireland, with broadly consistent usage. It is commonly contrasted with a condition subsequent, which gives a right to terminate but does not end the interest automatically. Determinable interests arise in drafting of freehold titles, leases, trusts, security interests and commercial contracts, where parties want an interest or right to persist only while a specified state of affairs continues. Practitioners must distinguish determinable provisions from void repugnant conditions and from unenforceable restraints on alienation, and consider registration, reverter or resulting trust consequences when a determinable interest ends.