An Alternative Trading System (ATS) is a platform that facilitates the buying and selling of financial securities outside traditional stock exchanges like the New York Stock Exchange or NASDAQ. ATSs provide a way for investors to trade directly with one another, often with lower fees and increased privacy. They can be used for various asset classes, including stocks, bonds, and derivatives. These systems are regulated by the Securities and Exchange Commission (SEC) in the United States to ensure fair trading practices. ATSs can take different forms, such as dark pools, which allow large trades to be executed without revealing the order to the public market, helping to minimize market impact.