The effect of surface roughness on the thermal microwave emission of soils has been studied with the Kirchoff approach. A model is presented that includes both the coherent and incoherent reflectivities of the rough surface. It is demonstrated that both the coherent and incoherent terms must be included in order for theroretical computations to provide good agreement with the experimental data, especially for wet soil, where surface roughness causes a dramatic increase in brightness temperature. In addition, the model including both the coherent and incoherent terms allows one to sue the physically measured surface height deviations of the rough surface in the model. Previous models have required that an effective height, not equal to the actual surface height measurements, be used in order for theoretical computations to match measurements.