What index tracks the performance of 500 large US companies?

The index that tracks the performance of 500 large U.S. companies is the S&P 500.

The name stands for Standard & Poor’s 500. It was introduced in its modern form on March 4, 1957, and is maintained by S&P Dow Jones Indices. The index is designed to represent leading large-cap companies traded on U.S. stock exchanges and covers roughly 80% of the U.S. equity market by capitalization.

Despite its name, the S&P 500 can contain more than 500 stocks because some companies have multiple share classes. Its weighting is based on free-float market capitalization, so larger companies generally have a greater effect on its movements than smaller constituents. A committee selects and reviews the companies rather than admitting firms solely because they are the biggest.

The Russell 2000 focuses on smaller companies, the Nasdaq Composite includes companies listed on Nasdaq, and the Dow Jones Industrial Average contains only 30 prominent companies. The S&P 500 is therefore often used as a broad snapshot of large-company American equities.