A disc brake lock used principally with motorcycles has a body (10) with a U-shaped slot (13) to receive an edge of the disc. A locking pin can be moved to extend through a hole in the disc to fix the lock to the disc. A limb (11) of the U-shaped body (10) that is on the outside when the lock is clamped to the disc contains an alarm system (14) housed in a chamber (28) that cannot be accessed through that limb. The other limb (12), which is between the disc and the wheel, contains a removable cover (34) that, for extra security, can only be opened when the locking pin (16) is retracted. The alarm system (14) is inserted into the chamber (28) through the other limb (12).