The S&P MidCap 400 Index, more commonly known as the S&P 400, is a stock market index from S&P Dow Jones Indices. The index serves as a barometer for the U.S. mid-cap equities sector and is the most widely followed mid-cap index in existence. To be included in the index, a stock must have a total market capitalization that ranges from $1.4 billion to $5.9 billion at the time of addition to the index. The index was launched on June 19, 1991.