[1] The index started on 3 January 1984,[2][3] having been constructed by the London Stock Exchange to better reflect activity on the market.

[4][5] As late as 10 February, the Stock Exchange referred to the index as 'SE 100', cutting out the Financial Times who had not contributed to its construction.

[6] The new index allowed the Stock Exchange's own London Traded Options Market (LTOM) to launch an options contract derived from the FTSE's real-time data in May 1984, while competitors LIFFE were quick to coincide their launch of the futures contract.

[11] The combination of a new index, major privatisations, LIFFE tradable derivatives, and promotion by the Financial Times led to the FTSE 100 becoming the most widely used indicator of whether the UK stock market was rising or falling.

[11] 2024 saw delistings from the London market in numbers not seen since the 2007–2008 financial crisis, with companies citing higher valuations and cheaper costs available by switching to NYSE.

Takeovers from private equity further reduced the available pool of companies that could be drawn on to populate the index, with Hargreaves Lansdown also set to delist.

Across the entire exchange there were five delistings for every new issue, with speculation at the close of the year around the future of index stalwarts British American Tobacco, Rio Tinto, and Shell.

[2] The FTSE 100 broadly consists of the largest 100 qualifying UK companies by full market value.

[15] However, many of these are internationally focused companies: therefore the index's movements are a fairly weak indicator of how the UK economy is faring and are significantly affected by the exchange rates of the pound sterling.

[1] The index consists of 20 ICB sectors, four of which had a market capitalisation exceeding £200 billion as of 31 December 2024[update].

These are Banks, Health Care, Industrial Goods and Services and Energy, which together account for approximately 48% of the index's capitalisation.

[18] Each calendar quarter, the FTSE's constituents are reviewed and some companies exit or enter the index, resulting in irregular trading volume and price changes as market participants rebalance their portfolios.

The free float adjustment factor represents the percentage of all issued shares that are readily available for trading, rounded up to the nearest multiple of 5%.

[19] FTSE futures contracts are traded on the Euronext Equities & Index Derivatives (EUREID) exchange.