The S&P 500 Index or Standard & Poor's 500 Index is a market-capitalization-weighted index of 500 leading publicly traded companies in the U.S. The index includes 503 components because three have two share classes listed.

It's not an exact list of the top 500 U.S. companies by market cap because the index includes other criteria. The S&P 500 index is nonetheless regarded as one of the best gauges of prominent American equities' performance and the stock market overall.