ASLEF is Britain's trade union for train drivers. Its 20,000+ members are employed in the train operating companies, the freight companies, London Underground and some Light Rapid Transport.

The first recorded meeting of the 'Associated Society of Locomotive Steam Enginemen and Firemen' was held on 7 February 1880 and its first monthly magazine was published eight years later. The union has seen its members working 12 or 16 hour days and six day weeks; it has fought major industrial disputes; it has seen nationalisation and privatisation; it has witnessed massive growth in the railways and the slashing cuts of Beeching. Throughout it all, it has continued to advance the opinions of its members with dignity, force and coherence that should be expected of the UK's foremost craft union.

Today the union's head office is located at 77 St John Street, London EC1M 4NN. The union's day to day direction is overseen by the Executive Committee. Each of its 8 regions has a District Organiser and an Executive Committee member, while the main negotiations with employers are undertaken by Company Council Representatives.

The union also has a retired members section and five representative committees representing members who are women, BAME, LGBT+, Disabled and Young members. It is affiliated to the Trades Union Congress and to the Labour Party.