The resonance phenomenon observed in Rayleigh wave scattering from surface-breaking cracks has been investigated using Freund's results on reflection of Rayleigh waves from an infinite crack edge. To model the crack as a section of acoustic waveguide, resonances are treated as standing waves in the length and depth directions. The model takes both faces of the crack into consideration, and this makes it possible to explain the observations of all order resonances in the length direction for excitation by a Rayleigh wave beam at normal incidence. Calculations are made for rectangular and half-penny shaped cracks and differences between the two cases are discussed.