The S&P 600 Index is a stock market index that measures the performance of 600 small-cap companies in the United States. It is part of the S&P Dow Jones Indices and is designed to provide a benchmark for small-cap stocks, reflecting their overall market performance. The index includes companies from various sectors, such as technology, healthcare, and consumer goods. It is widely used by investors and fund managers to gauge the health of the small-cap segment of the market and to make informed investment decisions.