A new approach to microwave eddy current testing for surface cracks in metals involves the use of ferromagnetic resonance in a small garnet crystal placed close to the surface to be tested. It is well known in this case that the induced eddy currents on the metal surface cause a strong displacement of the ferromagnetic resonance frequency. The presence of a crack on the surface produces, by disturbing the eddy current pattern, a perturbation of the resonant frequency as the garnet sample passes over it. Theoretical detectability criteria for the garnet probe are developed on the basis of a magnetic perturbation relation and compared with calculations of Rayleigh wave and plate wave backward scattering and intermode scattering at a half-penny surface crack. Some experimental results are also given.