Alternative Trading Systems (ATS) are platforms that facilitate the buying and selling of financial securities outside traditional stock exchanges like the New York Stock Exchange or NASDAQ. They provide a venue for traders to execute trades in a more flexible and often less regulated environment, catering to various market participants, including institutional investors and retail traders. ATS can include various types of trading venues, such as dark pools, which allow for anonymous trading, and electronic communication networks (ECNs) that match buy and sell orders. These systems aim to enhance market efficiency and liquidity while offering alternatives to conventional trading methods.