A modular intrusion detection system that includes a base unit having a primary intrusion detection device and a housing. The primary housing includes a base housing section and an outer cover. The base housing section includes a first mounting interface. A plurality of different secondary housing sections having a second mounting interface which can be attached to the first mounting interface of the base housing section are provided. The secondary housing sections may have a functional device, such as a CCD camera or a microphone, associated therewith. The camera or microphone may be activated to an alarm signal generated by the intrusion detection device. A plurality of different base units, each having an intrusion detection device, may also be provided wherein each of the base units is attachable to each of the secondary housings, and secondary devices associated therewith, to provide flexibility in assembling individual devices.