I found these monitors a brilliant idea especially for a very worried first time dad who would be in every 5 minutes checking on our new baby if we didn't have these. There is a base unit which stays in the babies room and an otpional sensor pad which you need to place on a flat surface underneath the mattress, we used a wooden board for this. The sensor pad then plugs into the base unit and you flick the switch to the correct setting, it beeps when it is working. If it detects no movement for 20 seconds an alarm sounds in both the base unit and the parents monitor which I don't think is a very good idea as it wakes your baby. I only ever had a couple of false alarms with it and that was when she was older and could move about a lot more, at this stage I just used it as a normal monitor and took away the sensor pad. You can sometimes get some inteference on them but usually it is relieved by unplugging electrical items around the units.