The S&P 500(also known as the Standard & Poor's 500), a registered trademark of the joint venture S&P Dow Jones Indices, is a stock index that consists of the 500 largest companies in the U.S. It is generally considered the best indicator of how U.S. stocks are performing overall. From another angle, the S&P 500, as an index, is a statistical measure of the performance of America’s 500 largest stocks. In this context, the S&P 500 is a common benchmark against which portfolio performance can be evaluated.