The S&P 500 is a stock market index that tracks the performance of 500 of the largest publicly traded companies in the United States. It includes well-known businesses across many industries, giving a broad snapshot of how the overall U.S. stock market is doing. Investors often use the S&P 500 as a benchmark to measure how other investments are performing. Because it represents a large portion of the market, it’s also commonly used in index funds and other passive investment strategies.