You have probably seen the headlines: “The S&P 500 closed higher” or “The FTSE 100 fell sharply.” For investors, these indices are not just numbers scrolling across a news feed; they are the benchmarks used to measure portfolios, compare performance and understand where money is flowing across major markets.
A stock market index is a collection of shares selected to represent a particular market or group of companies. The FTSE 100, for example, follows the performance of 100 of the largest companies listed in the UK, while the S&P 500 tracks leading US companies across multiple sectors. Instead of analysing thousands of individual businesses, investors use indices as a quick reference point for the wider market.
However, an index is only as representative as its construction. Companies are weighted differently, meaning a handful of the largest businesses can have a significant influence on returns. This means an index can rise even when many individual shares are falling, or fall despite strength across parts of the market.
The FTSE 100 vs. The FTSE 250
For UK investors, the FTSE 100 is the index that dominates the headlines, but it is not a perfect measure of the British economy. It tracks the 100 largest companies listed on the London Stock Exchange, yet many of its biggest constituents are international businesses such as Shell, HSBC and Rio Tinto. Their revenues come from around the world, with a large share earned in foreign currencies rather than sterling.
This means the FTSE 100 can rise when the pound falls, as overseas profits become more valuable when converted back into sterling. The index is heavily weighted towards established industries such as banking, energy and mining, giving it a very different profile from the UK economy most people experience day to day.
For a closer view of domestic UK businesses, investors often look towards the FTSE 250. These mid-cap companies tend to generate more of their revenues within the UK, making the index more closely linked to consumer spending and domestic growth. It has historically offered greater growth potential than the FTSE 100, although investors must accept larger price swings along the way.
Market-Cap Weighting
Most major indices, including the S&P 500 and the Nasdaq, are market-capitalisation weighted. This means the larger the company, the more influence it has over an index.
For example, in 2026, a tiny handful of tech giants – such as Nvidia, Palantir, Microsoft, Alphabet, and Meta – dictate the movement of the entire US market.
If these five or six companies have a bad day, the index crashes, even if the other 490 companies are performing well. When you buy a diversified S&P 500 tracker, you are, in reality, making a concentrated bet on Big Tech.
The Price-Weighted Oddity
Then there is the Dow Jones Industrial Average (DJIA). Unlike most indices, it is “price-weighted.” In this system, a company with a share price of £200 has twice the influence of one priced at £100, regardless of which business is actually larger or more important. Most serious investors see the Dow as a quirky 19th-century construct trying to track a 21st-century economy. It survives because of tradition, not because it gives an accurate picture of the market.
Using Indices as a Tactical Tool
Benchmarking: The index is your opportunity cost. If your hand-picked portfolio isn’t consistently beating the FTSE All-Share or the S&P 500, you are working hard for a lower return than a cheap passive fund would provide.
Currency Exposure: As a UK investor, buying a US index means you are also speculating on the GBP/USD exchange rate. If the S&P 500 goes up 5% but the Dollar drops 5% against the Pound, your profit in Sterling terms is zero.
Sector Awareness: If you hold a FTSE 100 tracker, you are heavily exposed to commodities and finance, while if you want exposure to innovation, you’ll need to look towards the Nasdaq or specific thematic indices. Stock market indices are not lines drifting across a screen; they are benchmarks that shape performance, fund flows and investor expectations.
If you buy a tracker, you are not owning the market in some vague sense. You are buying a specific slice of it, defined by geography, sector weightings and a handful of dominant companies. Know what sits under the bonnet before you commit capital.
Indices Compared
Each index captures a different section of the market. This table breaks down the structural differences between the major indices you are likely to track in your ISA or brokerage account.
| Index | Market Focus | Weighting Style | Primary Sector Bias | Indie Investor Insight |
|---|---|---|---|---|
| FTSE 100 | 100 Largest UK-listed firms | Market Cap | Energy, Mining & Banking | A global index in; 75% of revenue comes from outside the UK. |
| FTSE 250 | Mid-sized UK companies | Market Cap | Domestic Retail & Services | The true barometer of the UK. Usually offers higher growth than the FTSE 100. |
| S&P 500 | 500 Largest US firms | Market Cap | Technology & AI | Highly concentrated; the top 7 companies often outweigh the bottom 400 combined. |
| Nasdaq 100 | 100 Largest US non-financials | Market Cap | Pure Tech & Biotech | High-octane growth, but extremely sensitive to interest rate hikes. |
| Dow Jones | 30 blue chip US giants | Price-Weighted | Industrial & Consumer | A historical relic. A high share price gives a company more power than a larger valuation. |
What Is A Stock Exchange?
Stock indices are created and maintained by stock exchanges, which are marketplaces where investors buy and sell assets such as shares, bonds and ETFs.
Traditionally, stock exchanges were physical locations where traders would meet to make transactions. Today, most trading happens electronically on online platforms.
Stock exchanges serve several purposes. One key role is helping companies raise money by selling shares of ownership to the public. When investors buy these shares, they provide capital that companies can use to expand and develop their business.
At the same time, stock exchanges offer a marketplace where investors can buy and sell shares of listed companies. Investors hope the value of their shares will increase over time so they can sell at a profit. Share prices fluctuate constantly, driven by supply and demand – the balance between how many people want to buy and how many want to sell.
Investment Options
Stock exchanges offer a wide range of investment options beyond just traditional stocks and shares. Here are some other instruments you can trade:
Bonds: These are essentially IOUs issued by companies or governments. By buying a bond, you’re essentially loaning money to the issuer in exchange for a fixed interest rate payout over time and the return of your principal amount at maturity.
Exchange-Traded Funds (ETFs): These are baskets of securities that track a particular index, sector, or commodity. So, instead of buying individual stocks, you can buy an ETF that represents a whole group of them. This offers diversification and can be a good way to invest in a specific market segment.
Derivatives: These are contracts that derive their value from an underlying asset, such as a stock, bond, commodity, or currency. Examples include futures contracts, which lock in a price for buying or selling an asset at a future date, and options contracts, which give you the right (but not the obligation) to buy or sell an asset at a certain price by a certain time.
In some cases, there may be exchange-traded commodities (ETCs) available. These are similar to ETFs but track the price of a physical commodity like gold, oil, or wheat. They offer a way to invest in commodities without having to take physical possession of them.
Global Stock Exchanges
Global stock exchanges act as the beating heart of the investing world, facilitating the buying and selling of company shares across every sector and industry. Some of the most prominent stock exchanges that drive global capital markets include:
- New York Stock Exchange (USA)
- NASDAQ (USA)
- London Stock Exchange (UK)
- Tokyo Stock Exchange (Japan)
- Shanghai Stock Exchange (China)
- Hong Kong Stock Exchange (Hong Kong)
- Toronto Stock Exchange (Canada)
- Bombay Stock Exchange (India)
- Frankfurt Stock Exchange (Germany)
- Paris Stock Exchange (France)
- Madrid Stock Exchange (Spain)
- Amsterdam Stock Exchange (Netherlands)
Oldest Stock Exchange
The title of the oldest stock exchange goes to the Amsterdam Stock Exchange (currently called Euronext Amsterdam). It emerged in 1602, shortly after the founding of the Dutch East India Company. This company’s shares were traded regularly, creating a secondary market for them, which is considered the foundation of the modern stock exchange.
Biggest Stock Exchange
The biggest stock exchange in the world by market capitalisation is the New York Stock Exchange (NYSE). Market capitalisation refers to the total value of all the companies listed on the exchange. The NYSE has held this title for a long time, surpassing the London Stock Exchange.
Listing Requirements
Each stock exchange has its own listing requirements that companies must meet to be traded on their platform. These requirements focus on a company’s financial health, size, and transparency.
Key Requirements
- Minimum Shares Outstanding: Exchanges mandate a minimum number of shares a company must have available for trading. This ensures sufficient liquidity for investors.
- Market Capitalisation Threshold: Companies must reach a certain market cap, the total value of their outstanding shares. This indicates a company’s size and potential for growth.
- Financial Track Record: Exchanges often require a minimum level of profitability over a set period. This reassures investors of the company’s ability to generate sustainable earnings.
Examples
- New York Stock Exchange (NYSE): The NYSE demands a high bar: at least 1.1 million shares worth $40 million combined and a minimum of $10 million profit over three years.
- London Stock Exchange (LSE): The LSE’s main market has different criteria. Companies need a minimum market cap of £700,000, three years of audited financial statements, and at least 25% of shares publicly available (free float). Additionally, they must demonstrate sufficient working capital for at least a year after listing.
