The presented paper tries to establish a compact theory of the hitherto only slightly explored effect of modulated electro-magnetic microwave field detection in a glow discharge. Apart from theoretical analyses aimed at maximum possible generality and applicability also for other types of discharges, the paper presents the most important results of experimental verifications. The whole theory is divided into two parts: the macroscopic part where the discharge is studied as a disrupted macroscopic system, and the microscopic one dealing with the change in the discharge structure caused by a microwave field. Apart from discovering the mechanism of the detection effect, the theory also provides possibilities of application for plasma diagnostics.