A stock index is a group of companies clustered together, often providing a condensed snapshot of a market segment.

The S&P 500, managed by the same company, is another major stock index

Also run by S&P Dow Jones Indices, the S&P 500 tracks 500 large US firms using market-value weighting. Many analysts view it as a more accurate gauge of the American economy.

The Dow Jones Industrial Average is a stock market index

The Dow Jones Industrial Average tracks 30 large U.S. companies from all major U.S. sectors except for utilities and transportation. It's considered a bellwether of the U.S. economy.

Investors rely on the S&P 500 as a benchmark for the state of the economy

Since the index represents a broad range of diverse industries, it's become a proxy for the market itself and a common benchmark for investors to compare returns and performance patterns against. Generally speaking, when someone says a stock is under- or out-performing the market, that actually means under- or out-performing the S&P 500. The S&P 500 is exchange-agnostic, and includes companies listed on both the NYSE and Nasdaq.

The Nasdaq Composite index includes all of the companies traded on the exchange

This 3,000+ company index stands in contrast to the Nasdaq 100, which only includes 100 of the most influential non-financial companies traded on the exchange. These are the two most commonly used indices for Nasdaq investments.

The Dow Jones Industrial Average vs. the S&P 500: Which one is a better representation of the economy?

The Dow and S&P 500 are both indexes meant to provide a broad representation of the economy. The Dow has 30 companies and is weighted by share price, while the S&P 500 has 500 different companies weighted by market capitalization. Since the indexes are so different, investors don't always agree on which index is a better representation of the economy—though the two move in the same direction more than 90% of the time.

Index funds are typically structured as either mutual funds or exchange-traded funds, but investors can manually recreate an index through 'direct indexing.'

The Dow now tracks 30 major US companies across various industries

Since 1928, the DJIA has included 30 firms. Today's group spans technology, healthcare, and finance—with Apple, Microsoft, JPMorgan Chase, Pfizer, and Coca-Cola among its most recognized components.

The stock market, explained in 3 minutes

The stock market consists of a set of exchanges and other venues where shares of public companies are bought and sold. The Nasdaq and the New York Stock Exchange are two of the most well-known exchanges in the US, while major exchanges also operate around the world, including in Tokyo, Shanghai, London, and São Paulo.

'The stock market' can be loosely defined as a set of exchanges and other places where shares of public companies are bought and sold.

The Dow remains a shorthand for America’s economic health

For over a century, the Dow Jones Industrial Average has been one of the most closely watched market barometers. Its rises and falls dominate headlines and politics, shaping how Americans perceive the strength of their economy.

Before the S&P 500 launched in 1957, Standard and Poor's had a daily index with just 90 companies

Unlike previous indexes from S&P, the S&P 500 could be computed in real time using computer technology that tracked stock tickers across stock exchanges. This innovation is partly what led to 500 companies being listed on the index, a huge increase from the 90 companies in S&P's flagship index when the company had to do all the analysis by hand.

The Dow Jones Industrial Average was created in the aftermath of a financial crisis

Charles Dow introduced the index in 1896, following the Panic of 1893—a severe economic downturn. The DJIA offered investors a straightforward tool to track recovery and gauge the health of America's emerging industrial economy.

The Dow Jones Industrial Average was created in 1896

Charles Dow introduced the DJIA on May 26, 1896, as a simple average of 12 industrial stocks. Although it became the nation’s most enduring market index, none of its original companies remain in the Dow today.

Dow components are chosen by a committee, not a formula

Unlike many indices, the Dow has no strict rules for inclusion. A small committee at S&P Dow Jones Indices uses judgment—favoring companies with strong reputations, steady growth, and broad investor interest.

Visualizing decades of stock market performance

Stock markets and the indexes that track their biggest companies tend to ebb and flow, with long growth periods punctuated by shorter recessions. Track the 60-year history of one of the most common stock market indexes, the S&P 500, with this data visualization. The chart highlights the long-term durability of markets despite appearance of short-term volatility.

The Nasdaq 100 index tracks the exchange's 100 largest non-financial companies

This index was created in 1985, over a decade after the Nasdaq first launched. The companies on the index must meet certain criteria, including having a minimum average daily trading value of $5M.

A 'large-cap' company has a market capitalization of more than $10B

The S&P 500 is primarily composed of large-cap companies. The idea is that the index provides a big-picture view of the economy by assessing the performance of many of the largest public companies across different industries. The median market capitalization of companies listed on the S&P 500 is around $40B.

In late 2025, the top 10 companies in the S&P 500 made up more than 40% of the index's total value

This raised concerns for investors about the index's risk exposure. As many large tech companies continued posting record highs, analysts argued that the top-heavy index was becoming less representative of the economy as a whole.

See every company in and out of the Dow since 1928

The index has had 30 companies since 1928. See each company’s tenure on the index and what firms were added or replaced by year and industry.

The S&P 500 is composed of 500 companies, but has more than 500 total listings

Since some large companies have dual-class stock structures (like Alphabet and Fox Corporation), there are multiple listings on the S&P 500 for the same company. S&P Global has banned and then reversed bans on companies with dual-class structures multiple times since first allowing dual-class stocks in 2014.

Watch how the S&P 500 is actually calculated

After accounting for each company's float-adjusted market capitalization (a metric that combines the price of a company's stock, the total number of shares, and the stock's float, which means how many shares are actually available), the S&P 500 formula uses a divisor to create the index number. However, what the divisor is and what it represents are secrets.

S&P Global, the company that makes the S&P 500, is listed on the S&P 500

When the S&P 500 index was released in 1957, it was helpful for investors, but not necessarily profitable. S&P Global now makes more than $1B a year just from selling its index data. Alongside its other services, like credit ratings, S&P Global makes more than $14B a year.

The Dow’s price-weighted formula makes stock price matter more than size

Unlike most indices, which weigh companies by market value, the DJIA is price-weighted—meaning higher-priced stocks have more influence on its movements. As a result, a company’s share price matters more than its overall size.

Explore a step-by-step guide for getting started as a retail investor

Instead of purchasing individual stocks and bonds, new investors may want to start in funds that track an index—like the S&P 500—or that hold a group of themed stocks.

Why some say the S&P 500 has effectively become an AI fund

The S&P 500 is a market-cap-weighted index, which means companies with larger market capitalizations have a larger impact on the index's overall performance. Many of the top companies in the index are currently tech companies—including Nvidia, Microsoft, Alphabet, and Apple—that all have large investments in artificial intelligence. As a result, some analysts are concerned that the index is overly exposed to risky AI investments.

The Dow Divisor keeps the index consistent through time

To adjust for stock splits, mergers, and replacements, the DJIA uses a figure called the Dow Divisor. Originally valued around 12 in 1896, and now less than one, it ensures today’s index levels remain comparable with the past.

The S&P 500 selection committee has strict rules around adding a company to the index

While the decision to add a company to the index is ultimately up to the selection committee, any prospective company has to meet some criteria to be considered. To join, the company must have been profitable for the last four quarters, must have a market capitalization exceeding $22.7B, and meet certain requirements around tradeable stock availability (or float).

Some argue that the S&P 500 is actually an actively managed index

Some critics of the S&P 500's selection process argue that human involvement makes it an actively managed index. S&P Global maintains that while a human selects the stocks in the index, those decisions are not made with the intent of maximizing performance, but rather, to provide a snapshot of the largest companies across different industries.

Listen to the methods and strategy of the secretive S&P 500 selection committee

Stocks are added to the S&P 500 by a secret selection committee that meets regularly to assess which companies can remain in the index, which companies need to be added, and which companies no longer meet the criteria. The members of the committee used to be public, but S&P Global decided to keep its members confidential after companies sent packages, letters, and gifts to try to make it into the index.

Millions of Americans will be invested in SpaceX because of the fast-tracked process to list SpaceX on the NASDAQ 100, and, subsequently, the index funds that track it.

Why does the stock market matter?

Open any newspaper, turn on your local TV news station, or listen to a daily business radio segment, and you’re bound to hear about how the stock market is performing. Netflix created a 17-minute (free) explainer that dives deep into the nitty-gritty of how we got here.

The 'Big Three' asset managers—Vanguard, State Street, and BlackRock—collectively manage more than $30T in assets and are the largest shareholders in roughly 88% of all the companies listed on the S&P 500.

Critics argue the Dow no longer reflects the modern economy

With only 30 companies and a price-weighted formula, skeptics say the DJIA underrepresents vast sectors of the market. They claim it is outdated, though its close correlation with the broader S&P 500 keeps it relevant.

Read about the evolution of the S&P 500 and its recession recovery

Since the S&P 500 is considered a bellwether index (meaning it represents the health of the US economy), its value is important for investors in both bull and bear markets. The S&P 500 has been impacted by recessions and downturns, from the 1970s oil crisis to the dot-com crash of the early 2000s.

Vanguard Group founder Jack Bogle created the first index fund, the Vanguard 500, in 1976—but the fund didn't take off until the 1990s.

The Nasdaq's 'Magnificent 7' have an outsized impact on the Nasdaq 100's performance

These seven companies are Alphabet, Amazon, Apple, Meta, Microsoft, Nvidia, and Tesla. They're often responsible for over 40% of the Nasdaq 100 index's gains, but the exact percentage varies from year to year.

40 key stock market terms, explained

Liquidity. Forex. Capitalization. Blue chip stocks. These are just a few pieces of stock market vocabulary most people who don't spend all day on a trading floor might not know. Thankfully, we found a fun infographic that makes it easier to learn what 40 key stock market terms mean and why each of them matters.