Abstract: A high sensitivity smoke detector comprises a cylindrical housing incorporating a sampling chamber through which air to be sampled is forced. A modulated light source, such as a laser, directs modulated light through the chamber to a beam dump. The light beam is offset from the axis of the housing by an acute angle of between 15 and 50 degrees. If any smoke particles are positioned within a sampling volume, the light is scattered along a path defined by baffles to an axially positioned light sensor whose electrical output is passed through a phase-sensitive detection circuit which is referenced by the frequency at which the light source is modulated, so as to produce an output dependent on the light scattered by the smoke particles.