A cylinder lock is mounted in an aperture in a panel by means of a retaining ring which has resilient teeth to grip a part of the lock housing which protrudes from the panel. A further ring defining a non-circular aperture is provided in the opening of the panel to co-operate with flats on the lock housing and prevent rotation of the housing in the panel. The lock comprises a main body portion and a hollow spigot projecting from the main body portion and a key receiving member disposed in the spigot wherein the spigot defines an opening at an open end of the spigot remote from the main body portion. The key receiving member defines a key slot which is adjacent the open end of the spigot. The main body portion overlaps a marginal portion of the panel adjacent the panel opening defined thereby and the spigot extends through the aperture from the main body portion and projects from the panel at a second side thereof.