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What Is a Stock Index? The S&P 500, Dow and More

For investors, an index tracks the performance of a group of assets. The S&P 500 is a well-known stock market index.

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Head of Content, Investing & Taxes

Arielle O’Shea leads the investing and taxes team at NerdWallet. She has covered personal finance and investing for nearly 20 years, and was a senior writer and spokesperson at NerdWallet before becoming an editor. Previously, she was a researcher and reporter for leading personal finance journalist and author Jean Chatzky, a role that included developing financial education programs, interviewing subject matter experts and helping to produce television and radio segments. Arielle has appeared on the "Today" show, NBC News and ABC's "World News Tonight," and has been quoted in national publications including The New York Times, MarketWatch and Bloomberg News. She is based in Charlottesville, Virginia.

An index tracks the performance of a group of preselected investments, such as stocks. For example, the S&P 500 index tracks the performance of 500 of the largest U.S. companies. Investors gauge the performance of stocks, bonds or mutual funds by comparing them with the performance of an index.

The S&P 500 and the Dow Jones Industrial Average (DJIA or "the Dow") are two of the most well-known stock market indexes. While these indexes track the broad market and large-company stock movements, other indexes may track only a certain industry or sector of the market.

Stock index examples

Here are some details on common indexes that investors — as well as a range of index funds and ETFs — follow:

1. S&P 500 Index: The S&P 500 tracks the market moves of around 500 of the largest publicly traded companies in the U.S. The index is a representation of leading companies in leading industries and is capitalization-weighted, which means each stock is weighted proportionately to the company’s market capitalization. For example, a company whose total shares are valued at $100 billion is weighted more heavily than a $10 billion company. (Learn how to invest in the S&P 500.)

2. Dow Jones Industrial Average: The DJIA follows the performance of the 30 largest U.S. companies, also known as “blue chip” stocks. Market capitalization is not considered in this index. There are a few different Dow ETFs that track the index in different ways, depending on your investment goal.

3. Bloomberg Barclays U.S. Aggregate Bond Index: Also known as the Agg, this is a broad index that tracks the U.S. bond market. The index measures the investment grade, U.S. dollar-denominated, fixed-rate taxable bond market.

4. Nasdaq Composite Index: The Nasdaq tracks the performance of more than 3,000 companies, with a heavy focus on technology.

5. Russell 2000 Index: This index tracks 2,000 smaller — also known as small-cap — companies. These generally have market capitalizations between $300 million and $2 billion.

» Explore further: What are the best index funds?

Brokerage firms

on Charles Schwab's website

on E*TRADE's website

on Vanguard's website

on Fidelity's website

Index investing: Index funds and ETFs

Individual stocks and actively managed mutual funds attempt to “beat” the market — that is, perform better than their benchmark indexes. But these attempts often fail, and more investors are using passive investing strategies: index funds or exchange-traded funds that aim to mirror broad-market or sector performance rather than beat it.

Index funds and ETFs are funds that hold stocks that are representative of an entire index, such as the S&P 500, so that the performance rises and falls alongside that benchmark index. As index values tend to rise over time, index funds and ETFs have become an important way that investors build long-term wealth.

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Learn more about

How to invest in index funds

How to invest in ETFs

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