Switch on any financial news programme, open any newspaper business section, or glance at a trading platform, and within seconds you will encounter references to indices. The FTSE 100 closed higher. The S&P 500 hit a new record. The Nasdaq fell sharply on technology selling. These terms get reported as though everyone already understands them. But if you are new to financial markets, you might reasonably be asking: what are indices and why does everyone talk about them as though they carry such importance?
The answer starts with a practical problem. Stock markets are complex places. The London Stock Exchange alone lists hundreds of companies. The New York Stock Exchange and Nasdaq together list thousands. Trying to understand how the market as a whole is performing by looking at individual share prices is not practical; the numbers would be overwhelming and often contradictory, with some stocks rising while others fallsimultaneously. What financial markets needed was a way to summarise overall performance in a single number that could be tracked and compared over time. That is exactly what an index provides.
A stock market index is a measure of the performance of a selected group of stocks. The companies included are chosen according to specific criteria, and their individual price movements are combined into a single figure using a defined calculation methodology. When the index rises, it means that, on balance, the shares within it have increased in value. When it falls, they have declined. The absolute level of the index number is somewhatarbitrary; what matters is whether it is higher or lower than it was yesterday, last month, or last year, and by how much.
The FTSE 100 is the best-known example in the UK. The Financial Times Stock Exchange 100 Index represents the 100 most highly capitalised companies listed on the London Stock Exchange. It was launched on 3 January 1984 at a base level of 1,000. As of May 2026, the index has risen to around 10,267 points, remaining approximately 19 per cent higher than it was a year ago. When people in Britain refer to how the market isdoing, they almost always mean the FTSE 100. Its performance is treated as a proxy for the health of British business, though that analogy has important limitations.
The way an index is calculated has a significant impact on what it actually represents. The most common method used by major indices including the FTSE 100 and the S&P 500 is market capitalisation weighting, whereeach company’s influence on the index is proportional to its total market value. The largest companies by market capitalisation have the greatest effect on where the index moves. This means that when a handful of giantcompanies have a bad day, the whole index can fall significantly even if the majority of the other constituents are holding steady or rising.
Different indices serve different purposes and capture different parts of the market. The FTSE 250 covers the 101st to 350th largest companies on the London Stock Exchange and is generally considered a betterbarometer of the domestic UK economy than the FTSE 100, because it contains a higher proportion of companies that earn most of their revenue within Britain. Many FTSE 100 constituents are large multinationals whosefortunes depend more on global conditions than domestic ones. Shell and BP, for example, respond primarily to the oil price rather than to what is happening in the British economy.
Globally, the most closely watched indices include the S&P 500 in the United States, which covers 500 large American companies and is widely considered the most important gauge of US stock market performance. The Dow Jones Industrial Average, one of the oldest indices in the world, tracks just 30 large American companies. The Nasdaq Composite is heavily weighted toward technology stocks and has been one of the most volatile major indices in 2026 as artificial intelligence enthusiasm and profit-taking have alternated in driving sentiment. The DAX 40 in Germany is the main European benchmark, covering the 40 largest companies on the Frankfurt Stock Exchange.
One important reason indices matter for traders rather than just long-term investors is that they can be traded directly as financial instruments. Through products such as contracts for difference and exchange-traded funds, traders can take positions on the direction of an entire index rather than picking individual stocks. Trading an index offers a degree of built-in diversification; a single company can collapse entirely, but an index of 100companies absorbs individual failures without being destroyed. In 2026, for instance, a significant number of individual S&P 500 stocks experienced drawdowns exceeding 40 per cent during a volatile period in the first quarter, yet the index itself declined far less than that because losses in some constituents were offset by gains in others.
Index constituent changes also create trading opportunities. The composition of major indices is reviewed quarterly, and when a company is added to or removed from an index, it triggers mechanical buying or selling by the large index-tracking funds that must adjust their holdings to match the index. Traders who anticipate these changes can position themselves ahead of the institutional flows.
Understanding what indices are and how they work is genuinely foundational knowledge for anyone who wants to engage with financial markets intelligently, whether as a long-term investor building wealth over decades or as a more active trader looking to capitalise on shorter-term market movements. The numbers that scroll across financial news tickers every day are not arbitrary; they are the distilled story of how millions of individual price decisions aggregate into a single measure of collective market sentiment.
