ADT and SmartThings Team Up
Samsung SmartThings and ADT recently (10/2/2017) announced a partnership combining SmartThings's DIY smart home platform with monitored home security by ADT. This finally brings ADT continuous monitoring services to SmartThings, but requires specific ADT approved hardware.
New Control Panel
For starters, you will need to purchase the new ADT Security Hub touchscreen panel. This functions similarly to a traditional security panel for arming and disarming the system, but also includes built in battery and cellular connection backups. Additionally the hub functions as a SmartThings Hub, so there is no need for a standalone SmartThings Hub.
They also introduced new SmartThings ADT sensors. The hub will work with other "Works with SmartThings" devices, but only the new ADT sensors can be used to trigger monitoring events. This means that your existing Z-wave or Zigbee sensors would need to be replaced in order to use them to control SmartThings automations AND trigger monitored security alarms. Fortunately, the new motion and door/window sensors are competitively priced.
Bottom Line Opinion
While on the surface this is a great addition to the SmartThings ecosystem, it is very disappointing that only the SmartThings ADT sensors can be used for ADT monitoring. They also did not introduce a glass break sensor or a recessed door/window sensor (which I prefer even if the installation is more involved). Hopefully these additional sensors, and maybe even a standard for other manufacturers, will be released in the future.