One of the broadest indices that is tracked by investors, the Russell 3000 Index contains six times as many stocks as the S&P 500, the most popular barometer for the stock market. It's a capitalization-weighted index (current share price multiplied by number of shares outstanding) that was established on January 1, 1984.

Collectively, the Russell 1000 and Russell 2000 are known as the Russell 3000. All three indices are maintained by the FTSE Russell Company, which is a subsidiary of the London Stock Exchange Group.

Since the market capitalization of companies changes over time, FTSE Russell Group reevaluates the companies in its indexes every May to ensure accuracy and then rebalances the indexes every June against all newly eligible companies that have had an initial public offering (IPO).

When the indexes are reconstituted, the breakpoints among large-cap, mid-cap, and small-cap stocks are redefined to ensure market changes that have occurred in the preceding year are captured.

The last Friday in June, when the Russell rebalancing takes place, has often been one of the highest-volume trading days of the year, due to institutional investors and funds that track the Russell indexes adjusting their holdings to reflect the updates.

Investors cannot invest directly in a stock index. While it is possible to individually purchase shares of each company represented in the Russell 3000 index, it’s simpler to purchase an exchange-traded fund (ETF) or an index fund that that tracks the performance of the target index.

Tracking the Russell 3000 is a good way to follow the aggregate U.S. stock market because the index covers small and large-cap stocks across various industries.