Fault divorce describes a form of divorce where one spouse must prove the other’s “fault” – such as adultery, unreasonable behaviour, desertion or other matrimonial offence – to obtain a decree or order for divorce. It contrasts with “no‑fault” divorce, where irretrievable breakdown is established without attributing blame.Across the UK and Ireland, the concept is rooted in legislation rather than being a free‑standing legal term. In England and Wales, and in Scotland, the Divorce, Dissolution and Separation Act 2020 has effectively removed fault‑based grounds, replacing them with a single ground of irretrievable breakdown established by a statement, so “fault divorce” is now largely of historic or comparative relevance only.In Northern Ireland and Ireland, fault remains practically significant. Statute still permits divorce based on conduct such as adultery or unreasonable behaviour (Northern Ireland) or behaviour rendering further cohabitation intolerable (Ireland), although separation‑based grounds are commonly pleaded.The term is used by family lawyers when advising on strategy, evidential requirements, pleadings and the costs and emotional impact of alleging fault, and when explaining the differences between historic and current divorce regimes.