FTSE Group is a British-based company that is responsible for maintaining and calculating several stock market indices, including the FTSE 100, FTSE 250, and FTSE All-Share indices. The company was formed in 1995 as a joint venture between the Financial Times (FT) and the London Stock Exchange (LSE), with the name FTSE being an acronym for "Financial Times Stock Exchange". In 2011, FTSE Group was acquired by the London Stock Exchange, making it a wholly owned subsidiary.

FTSE Group is responsible for determining the composition of the indices it calculates, which is done through a process known as index inclusion. Companies that meet certain criteria, such as being listed on the London Stock Exchange and meeting minimum size and liquidity requirements, are considered for inclusion in the indices. The exact criteria for inclusion varies depending on the index in question.

The FTSE 100 index, also known as the "Footsie", is the most well-known index calculated by FTSE Group. It consists of the 100 largest companies listed on the London Stock Exchange, ranked by market capitalization. The FTSE 250 index consists of the 250 next-largest companies after the FTSE 100, and the FTSE All-Share index includes all companies listed on the London Stock Exchange that meet certain criteria.

FTSE Group's indices are widely used as benchmarks for the performance of the UK stock market, and are used by investors to track the performance of their investments and make decisions about buying and selling stocks. The company also offers a range of other index-based services, including custom index design, index calculation and data services, and index-linked investment products.