ATS (Alternative Trading System)

An Alternative Trading System (ATS) is any organized electronic trading platform that matches buyers and sellers of securities but is not registered as a national securities exchange. ATSs are regulated under SEC Regulation ATS (adopted in 1998), which requires them to register as broker-dealers, file Form ATS with the SEC, and report all transactions to FINRA's Trade Reporting Facility.

Types of ATS

ATSs span a wide range of trading venues: Dark pools are the most commonly discussed type — private trading venues where institutional investors trade large blocks away from the public order book. Orders are invisible before execution and reported post-trade. Electronic Communication Networks (ECNs) are automated systems that display orders and match them electronically; unlike dark pools, ECNs typically publish their order books and provide pre-trade transparency. Crossing networks match large institutional block trades at midpoint prices, minimizing market impact without routing through public exchanges.

ATS vs National Exchange

The core regulatory distinction is that national exchanges (NYSE, Nasdaq, CBOE) perform a price discovery function for the public — they publish live order books and are required to report best bid and offer prices to the consolidated tape in real time. ATSs have no obligation to provide pre-trade transparency. They are venues for execution, not venues for price discovery. The implication for traders: prices formed in ATSs are influenced by, but do not directly contribute to, the public NBBO.

ATS Volume in US Equities

ATSs collectively account for approximately 40–42% of total US equity trading volume. The majority of this is dark pool volume, with ECNs contributing a smaller share. The fragmented execution landscape — dozens of ATSs competing with each other and with public exchanges — is why order routing algorithms are so important: the best execution for any given order depends on which venue offers the best price and liquidity at that moment.

Why ATSs Matter for Signal Detection

The post-trade reporting requirement for ATSs creates the data stream that dark pool monitoring tools aggregate. Platforms like TradeAI News monitor FINRA ATS post-trade data to detect statistically unusual volume patterns — large institutional transactions that, when combined with catalyst events and options flow data, contribute to higher TMS scores for affected tickers.