Abstract: The present invention concerns a unique vibration detector for sensing movement or vibration of an associated object. The detector includes an infrared light transmitter positioned to direct light toward a reflective fluid mass such as, for example, mercury. A cooperating infrared receiver is positioned to receive at least a portion of the light reflected off the fluid mass. As the object and detector are vibrated, the surface of the fluid will be disturbed, thus varying the amount of light energy detected by the receiver. In the preferred embodiment of the invention, the fluid mass of mercury is contained within a cylindrical chamber having a relatively small diameter such that the upper surface of mercury provides a generally convex-shaped reflecting surface. The vibration detector can be utilized, for example, in a vehicle anti-theft system.