The FTSE 100 Index is a share index of the 100 largest companies listed on the London Stock Exchange. It is the leading share index in Europe. The index is maintained by the FTSE Group, which began as a joint venture of the Financial Times and the London Stock Exchange. The full title is "The Financial Times Stock Exchange Index of 100 Leading Shares."
Related indices are the FTSE 250 Index which lists the next largest 250 companies; the FTSE 350 Index which combines the FTSE 100 and 250, and the FT All Share Index which quotes all listed companies.
The FTSE 100 is a market-cap weighted index. The constituents of the index are determined quarterly. The largest companies in the FTSE 250 Index are promoted if their market capitalisations would place them in the top 90 firms of the FTSE 100 Index. As of 2004, the threshold is about 2 billion pounds.
At its creation in January 1984, the index had a base level of 1,000. As of February 2019, it had a level of over 7,000.
The largest 5 companies in the index are: Royal Dutch Shell; HSBC Holdings; BP; AstraZeneca; and GlaxoSmithKline.