The S&P 500 stock market index, maintained by S&P Dow Jones Indices, comprises 505 common stocks issued by 500 large-cap companies and traded on American stock exchanges, and covers about 80 percent of the American equity market by capitalization. The index is weighted by free-float market capitalization, so more valuable companies account for relatively more of the index. The index constituents and the constituent weights are updated and checked on regularly using rules published by S&P Dow Jones Indices. Although the index is called the S&P "500", the index contains 505 stocks because it includes two share classes of stock from 5 of its component companies.
Recent and announced changes to the list of S&P 500 Components
S&P Dow Jones Indices updates the components of the S&P 500 periodically, typically in response to acquisitions, or to keep the index up to date as various companies grow or shrink in value. Between January 1, 1963 and December 31, 2014, 1,186 index components were replaced by other components.