A bicycle lock is a physical security device used on a bicycle to prevent theft. It is generally used to fasten the bicycle to a bicycle stand or other immovable object.
An important difficulty in preventing the theft of a bicycle is that the wheels are easily detachable from the frame, and that unless both wheels and frame are secured, wheels can easily be carried away after being detached. The most secure locking method therefore is to lock the wheels and frame to each other and to an immovable object.
Locking devices vary in size and security; the most secure tending to be the largest, heaviest and least portable. Lesser equipment is used to deter attempts by less skilled and determined thieves. Thus like other security equipment, bicycle locks must compromise between security, portability and cost. Some are made of particularly expensive materials chosen for their acceptable strength and low density.