Alternative Trading Systems (ATSs) are SEC-regulated electronic trading systems that match orders for buyers and sellers of securities. An ATS is not a national securities exchange. However, an ATS may apply to the SEC to become a national securities exchange.
All current ATSs are “dark pools.” Dark pools are trading systems that allow their users to place orders without publicly displaying the size and price of their orders to other participants in the dark pool.
For additional information on ATSs, please read the SEC rules here. For a current list of ATSs, go here: http://www.sec.gov/foia/docs/atslist.htm.