Alternative Trading System(ATS) or Alternative Trading Platform (ATS) are terms used to describe a financial market structure, which is not a physical trading floor, but an electronic system. ATSs may either be an exchange that trades securities listed on another exchange (internalization) or an electronic communication network (ECN) between two traders.The main buyers and sellers of stocks, bonds and options on any particular stock exchange are large brokerages and asset management investment firms. However, these institutions do not usually buy and sell directly from each other. Instead, they trade using an ATS. All market participants that are not large brokerages or asset management investment firms must use the ATS to indirectly trade with one another.Regulators in the United States and Europe have scrutinized ATSs due to concerns about unfair competition and market manipulation. The SEC’s official definition of an ATS is as follows: “An alternative trading system is a trading system that does not consist entirely of orders transmitted by one party to another for execution at the same price.