The Air Accidents Investigation Branch (AAIB) investigates civil aircraft accidents and serious incidents within the United Kingdom, its overseas territories and crown dependencies. It is also the Space Accident Investigation Authority (SAIA) for the United Kingdom. The AAIB is a branch of the Department for Transport and is based in the grounds of Farnborough Airport, Hampshire.

Aviation accident investigation in the United Kingdom started in 1912, when the Royal Aero Club published a report into a fatal accident at Brooklands Aerodrome, Surrey.

The AAIB was established in 1915 as the Accidents Investigation Branch (AIB) of the Royal Flying Corps (RFC). Captain G B Cockburn was appointed "Inspector of Accidents" for the RFC, reporting directly to the Director General of Military Aeronautics in the War Office.

After the First World War, the Department of Civil Aviation was set up in the Air Ministry and the AIB became part of that department with a remit to investigate both civil and military aviation accidents.

Following the Second World War a Ministry of Civil Aviation was established and in 1946 the AIB was transferred to it, but continued to assist the Royal Air Force with accident investigations – a situation which has continued ever since.

After working under various parent ministries,^{[citation needed]} including the Department of Trade, the AIB moved to the then Department of Transport in 1983 and in November 1987 its name was changed to its current form, the Air Accidents Investigation Branch (AAIB). Latterly, the AAIB has become part of the reorganised Department for Transport (DfT) since 2002. In 2024, the AAIB was awarded the Lennox-Boyd Award by the Aircraft Owners and Pilots Association.

The AAIB has 57 employees.

These are: