Includes 9 sensors with magnetic component, a 10th magnetic piece, and 10th backing to sensor. Batteries and screws are not included. I recently purchased a home that had these left inside. I have not tested as I am not a Simplisafe customer. Discoloration visible on some units. No corrosion inside battery compartments. These are NOT compatible with the 3rd generation system. Items shipped next business day. Thanks for looking.