I bought the first one on the list... https://www.dealextreme.com/details.dx/sku.27127 You have to put a battery in it before attaching it to the frame of the bike. Once attached, you can't remove the battery. This is a little inconvenient for maintaining the battery, but it makes it more difficult to defeat. The alarm is small enough to be relatively inconspicuous so it might be overlooked. It also has three sensitivity settings. The most sensitive goes off if the bike is in a street rack with other bikes and someone is fiddling with their bike. In any case, for the price of the alarm, it''s better to have than not.