Full wave solutions are presented for horizontally polarized waves scattered by rough surfaces. Complete expansions and exact boundary conditions are used in the analysis. These solutions are compared with the physical optics solution based on the local tangent approximation of the fields at the irregular boundary. Expressions for the backscatter cross section for both horizontally and vertically polarized waves are also compared with earlier solutions to this problem. The reciprocity relationship and the suitability of the approximate impedance boundary condition are examined, and special consideration is given to excitation and scattering at grazing angles. This work on irregular ground effects on radio wave propagation can be applied to problems of communication, navigation, positioning, and active remote sensing.