ADT has been around for a long time - they were founded in 1874 and pioneered the current security monitoring business in 1920. They are the giant in the home and commercial security market with over 8 million customers and 18,000 employees in 200 locations.
The company has a checkered history - they were found guilty of monopolistic practices in 1964 which required them to change their business practices and pay fines and damages to the government, customers and competitors.
In 1997 Tyco acquired ADT and then spent $2B to acquire the #2 security business (Brinks Home Security/Broadview . . .