The S&P MidCap 400 is a stock market index that measures the performance of 400 mid-sized companies in the United States. These companies are selected based on their market capitalization, which typically ranges from $2 billion to $10 billion. The index is designed to provide a benchmark for investors looking to track the performance of mid-cap stocks. Investors often use the S&P MidCap 400 to gain exposure to a segment of the market that can offer growth potential while also balancing risk. The index includes companies from various sectors, making it a diverse representation of the mid-cap market.