The S&P 500 is a value weighted index published since 1957 of the prices of 500 large cap common stocks actively traded in the United States. The stocks included in the S&P 500 are those of large publicly held companies that trade on either of the two largest American stock markets, the New York Stock Exchange and NASDAQ. Almost all of the stocks included in the index are among the 500 American stocks with the largest market capitalizations.
After the Dow Jones Industrial Average, the S&P 500 is the most widely followed index of large-cap American stocks. The S&P 500 is considered a bellwether for the American economy, and is included in the Index of Leading Indicators — an American economic index intended to estimate future economic activity. Some mutual funds, exchange-traded funds, and other managed funds, such as pension funds, are designed to mimic the performance of the S&P 500 index. Hundreds of billions of US dollars have been invested in this fashion.