The objectives of this work are to develop an electromagnetic scattering model that can be used to predict the scattering from a crack in a conducting surface and to evaluate the feasibility of using this model in conjunction with microwave-measurement techniques to determine the dimensions of such a crack. Such a theory has been developed, and its derivation is presented. Theory and experiment are compared for rectangular slots measured at 100 GHz, and the agreement is found to be good. The necessary measurement protocol for determining the dimensions of a crack is discussed, and an example of inverting the measured data to determine the dimensions of a rectangular slot is presented.