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The radar polarization properties of lava flows in Hawaii (Kilauea) and Arizona (SP flow), and two play a surfaces (Lunar Lake, Nevada and Lavic Lake, California), are compared to the predicted behaviors of theoretical scattering models. At 5.7 cm and 24 cm wavelengths, Kilauea lava flows can be modeled by a combination of facet and diffuse (dipole-like) scattering. Scattering by rock faces on the scale of the radar wavelength is proposed to account for much of the facet return. The radar echoes at 24-cm wavelength from SP flow are, on average, consistent with entirely diffuse scattering, but there are regions within the flow where circular polarization ratios exceed unity, suggesting a coherent scattering effect. 68 cm data for the lava flows show evidence of radar penetration and volume scattering. The playa surfaces are characterized by polarization properties which in some cases are qualitatively consistent with the first-order small-perturbation model, but the echoes do not closely match the predictions of this model for any reasonable dielectric constant value. These results show that it may be difficult to construct invertible models for the polarization behavior of some surfaces (the playas), whereas for others (the Kilauea lava flows) the scattering properties can be successfully modeled. The first-order small-perturbation model is not appropriate for inverse modeling of most terrestrial lava flows, though very smooth surfaces on Venus may be amenable to the use of this model. High circular polarization ratios observed for SP flow, tentatively attributed here to coherent backscatter, may be analogous to Arecibo observations of high-reflectivity areas on Venus.