The S&P 500 (^SPX) index is a basket that contains the stocks of 500 Large-Cap corporations. Most of these corporations are American and they are largest public companies which are traded on the biggest US Stock Market Exchanges - on the New York Stock Exchange (NYSE) and Nasdaq Exchange. The S&P 500 index consists of 500 leading companies in the U.S. economy and is considered as the best single gauge of the U.S equities market. The S&P 500 index is the second most watched index after the DJI (Dow Jones Industrials Index).

The history of the S&P indexes goes back to 1923 when the S&P 90 (backed of 90 public companies) index was introduced and was published on the daily basis. An index based on the 423 public companies was also published, but on a weekly basis only. With development of the computer industry it became possible to calculate broader indexes in real time and on March 1957 the S&P 500 index was introduced to the world. Later the index has been extrapolated back in time.

Similarly to the Dow Jones Industrials Index (DOW 30) the companies included into the index are selected by the committee. The committee selects large publicly traded companies with high liquidity, those that represent different industries in the U.S. economy and are traded on the New York Stock Exchange and NASDAQ Exchange.

Initially the S&P 500 index was market value weighted which means that companies with greater total market value (price time's number of outstanding shares) would affect index price stronger than the companies with smaller total market value. In 2005 the index has been converted to the float weighted – only those shares that are available to public trading are counted.

There are various ways of investing into the S&P 500 index. An investor may purchase individual stocks from the S&P 500 index basket as well as the shares of the exchange traded funds (ETFs) that track the index. Standard & Poor's Depositary Receipts (SPDRs, commonly called "spiders"), is one of the most popular and most traded ETF (after QQQQ which tracks the NASDAQ 100 index). The SPDRs is traded under the SPY ticker symbol. There are other securities that allow to invest into the S&P 500 index: iShares S&P 500 (symbol:IVV), Rydex and ProFunds S&P 500 funds, options on the S&P 500, SPY options, S&P 500 e-mini futures and others.