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An electromagnetic (EM) bias model for microwave frequencies is derived using the physical optics scattering approximation with a sea surface model that includes the effects of hydrodynamic modulation and non-Gaussian long-wave statistics. Correlation of long-wave slopes with displacement and hydrodynamic modulation of short-wave amplitudes are the two major contributors to the EM bias. The fact that the bias is caused by physical processes with different dependencies on frequency and sea state parameters accounts for some of the difficulties encountered with previous bias modeling efforts. The frequency dependence of the bias is weak because the trends of the hydrodynamic and height-slope skewness contributions with respect to frequency are in opposition. The model provides theoretical support for previous empirical observations of weak correlation of relative bias with wind speed and strong correlation with root-mean-square long-wave slope. While the presence of wind-generated small-wave roughness is crucial to the portion of the bias caused by hydrodynamic modulation, the magnitude of the bias thus induced is largely independent of local wind speed. The bias model is validated by comparison to measured data from two tower radar experiments, the Gulf of Mexico experiment (1991-1992) and the Brigham Young University off-nadir experiment (2003). Both quantitative and qualitative agreements between model and measurements are observed.