This paper describes the estimation of parameters characterizing the vertical structure of vegetated land surfaces, from combined interferometric and polarimetric radar data. Physical models expressing radar observations in terms of parameters describing vegetated land surfaces are the foundation for parameter estimation techniques. Defining a general complex cross correlation enables the unified development of models for interferometry and polarimetry, including polarimetric interferometry. Three simple physical models in this paper express this complex cross correlation in terms of vegetation parameters: (1) a randomly oriented volume, (2) a randomly oriented volume with a ground return, and (3) an oriented volume. For the first two models the parameters include vegetation height, extinction coefficient, underlying topography, and another parameter depending on ground electrical properties and roughness. For the oriented volume, additional parameters depend on the refractivity, extinction coefficients, and backscattering characteristics of waves propagating along eigenpolarizations of the vegetation volume. The above models show that the interferometric cross-correlation amplitude and the polarimetric {HHHH/VVVV} ratio both change by about 1% per meter of vegetation height change, for experimental conditions typical of airborne and spaceborne interferometric radars. These vertical-structure sensitivities prompt a parameter estimation demonstration with two-baseline TOPSAR interferometric and zero-baseline polarimetric data from the Boreal Ecosystem-Atmosphere Study (BOREAS) Southern Study Area in Prince Albert National Park, Saskatchewan, Canada. The demonstrations show the feasibility of measuring vegetation height to better than 4.2 m, underlying topography to better than 6.5 m, and the ratio of ground-to-volume power to better than 10%, using interferometry and polarimetry, coupled with parameter-constraining assumptions, concerning the degree of surface roughness. This paper suggests that single-baseline and multibaseline fully polarimetric interferometry have the potential to obviate the need for such assumptions, thereby making parameter estimation more robust, accurate, and realistic. ¿ 2000 American Geophysical Union