What Is a Stock Market Index?

What Is a Stock Index?

A stock market index follows the performance of a particular set of stocks, which are shares of publicly traded companies. These stocks often represent a specific sector of the market and might reflect:

• A specific industry: The Nasdaq Composite, for example, is a market index that focuses heavily on tech stocks. It can provide investors with a general overview of how the technology sector as a whole is performing.
• A certain market capitalization: A company's market cap represents its market value. It's calculated by multiplying the current stock price by its total outstanding shares. Large-cap companies typically have a market value that exceeds $10 billion. The S&P 500 focuses on large-cap companies.
• Certain price points: High-priced stocks are the centerpiece of the Dow Jones Industrial Average. This is a price-weighted stock market index—so the higher a stock's price, the more it will impact the index.

It's important to note that a stock market index is different from a stock exchange, which is a marketplace for buying and selling stocks and exchange-traded funds (ETFs).

Learn more: What Is the Average Stock Market Return?

What Is the S&P 500?

The S&P 500 is one of the world's most well-known stock market indexes—and its performance is often seen as a reflection of the U.S. stock market at large. It's made up of about 500 of the largest, most stable companies in the United States. (Companies that have more than one share class have multiple listings, bringing the total just above 500.) The larger a company's market cap, the more it will influence the index.

As of February 2026, the following companies are among the most prominent within the S&P 500:

• Alphabet (Google's parent company)
• Amazon
• Apple
• Berkshire Hathaway
• Meta
• Microsoft
• Nvidia
• Tesla

For the last century, the S&P 500 has seen average annual returns of around 10%.

What Is the Dow Jones Industrial Average?

The Dow Jones Industrial Average, or Dow for short, is a price-weighted index that includes 30 blue-chip stocks. These are typically shares of large, well-established industry leaders. While market cap carries the most weight within the S&P 500, stock price has the biggest impact on the Dow. It covers all industries, excluding utilities and transportation.

As of February 2026, some of the top companies within the Dow include:

• Amazon
• Apple
• Chevron
• Coca-Cola
• Goldman Sachs
• Johnson & Johnson
• McDonald's
• Microsoft
• Nike
• Walmart
• The Walt Disney Company

The Dogs of the Dow investment strategy involves purchasing the top 10 Dow stocks that pay dividends. The goal is to outperform the index over time, but that's never guaranteed.

What Is the Nasdaq Composite?

The Nasdaq Composite is a large, technology-heavy index that includes more than 2,500 common stocks that are listed on the Nasdaq stock market exchange. Like the S&P 500, it focuses on market capitalization—so large-cap companies carry more weight. This index features a wide variety of companies within the technology sector.

Below are some major tech players you'll find within the Nasdaq Composite:

• Alphabet (Google's parent company)
• Amazon
• Apple
• Broadcom
• Meta
• Microsoft
• Nvidia
• Tesla

Are Index Funds Safe?

Index funds are considered low-risk investments. These are baskets of stocks that track a specific market index. The hope is that the fund will perform as well as its corresponding index, though returns are never guaranteed. These funds offer an affordable approach to investing that's less complicated than individually purchasing all the stocks within an index. They also have a reputation for long-term stability.

An exchange-traded fund (ETF) is a popular type of index fund. It's also possible to invest in indexed mutual funds. Either way, adding index funds to your portfolio can help with diversification and reduce investment risk.

Learn more: Pros and Cons of Index Funds

How to Invest in Index Funds

If index funds are in line with your investment goals and risk tolerance, you can take the following steps to invest:

1. Select an investment vehicle. That might be a brokerage account, a workplace retirement plan like a 401(k) or an individual retirement account (IRA).
2. Choose index funds to add to your portfolio. There are many ETFs and mutual funds that track different stock market indexes. For example, if you're looking for tech-centered investments, you might opt for a fund that follows the Nasdaq Composite. No matter what you choose, be sure to compare expense ratios and other fund fees.
3. Rebalance as necessary. The value of your investments can fluctuate over time, throwing off your asset allocation. This is why it's important to periodically rebalance your portfolio. This involves buying and selling assets to restore your desired risk tolerance.

Frequently Asked Questions

The Bottom Line

Stock market indexes focus on specific groups of stocks. The S&P 500, Dow Jones Industrial Average and Nasdaq Composite are among the most well known. Index funds allow you to invest in portfolios that aim to mimic an index's performance. That can be faster and less complex than buying all the individual stocks within a specific index.