ADT is a home security company that provides residential and commercial security products and services. The company was founded in 1874 by Edward Calahan, who invented the first stock ticker. ADT was acquired by the Tyco International conglomerate in 1997. In 2016, ADT was purchased by Apollo Global Management for $6.9 billion. As of 2019, ADT is the largest home security provider in the United States with over six million customers.