Clutter motion relative to the radar may seriously degrade the efficiency of moving target indication (m.t.i.) systems used in radio location. This problem arises in shipborne radar, airborne radar, or when the clutter is being carried by the wind. Although various methods have been devised to overcome the effect, the problem still exists because these have their inherent drawbacks. As another approach, the two-frequency m.t.i. system has been proposed occasionally in the past few years, but it seems that a more detailed consideration of this method is as yet not available in the literature. A basic theory of the two-frequency m.t.i. system, together with a discussion of its significant characteristics, is presented in this paper.