Alternative Trading System

A non-exchange trading venue that matches buy and sell orders for securities, often offering pre-trade anonymity.

An Alternative Trading System (ATS) is a non-exchange trading venue that matches buy and sell orders for securities. Regulated by the SEC in the United States as broker-dealers rather than national securities exchanges, ATS platforms offer an alternative marketplace to traditional exchanges like the NYSE or Nasdaq. They are widely utilized by institutional investors to trade large blocks of stock. Unlike public exchanges, many ATS platforms do not publicly display their order books or pre-trade quotes. These specific venues are commonly referred to as "dark pools." By keeping order sizes and pricing hidden until after a trade is executed, these systems allow institutional investors to buy or sell massive quantities of shares without alerting the broader market, which helps prevent adverse price movements. For retail investors, ATS platforms are an important structural component of modern market architecture. While retail orders are rarely routed directly to an ATS by the investor, broker-dealers frequently route retail order flow to these venues to find price improvement or lower execution fees. However, the fragmentation of liquidity across dozens of different ATS platforms can make it more complex to assess the true supply and demand of a security at any given moment.

An institutional fund wants to sell 500,000 shares of Company XYZ, which trades on a public exchange at $100. If the fund places this massive order on a public exchange, other traders will see the huge supply, likely driving the price down to $98 before the order can be filled. Instead, the fund routes the order to an ATS (dark pool) where the order is matched anonymously with a buyer at the midpoint price of $100, saving the fund $2 per share, or $1,000,000 in market impact costs.