A device and method for detecting the presence of living organisms in a structure or behind a wall or partition utilizes a microwave or radio-frequency one or more transceivers which generate separate and distinct interrogating signals and receives separate and distinct signals reflected from a structure and living organisms within it. The reflected signals received by each of the transceivers are processed, for instance by a microprocessor, so as to provide output signals that indicate the presence or absence of a living organism in the structure or behind wall or partition. The microprocessor distinguishes and differentiate signals from different living organisms and from false indication of the presence of living organisms, thereby enabling the detection of living organisms despite the existence of motion signals caused by non-living organism motion. Similarly, the device can distinguish between the biological characteristics, such as respiration rates, of targets to determine if the targets are of the type sought, for example, human targets as opposed to pets or insects.