The report reviews the basic properties and terrestrial surface responses of microwave sensors, both active (radar) and passive (microwave radiometer). Appropriate surface responses (bistatic coefficient, emissivity etc.) are defined and used to provide general formulas for the system response (receiver power, antenna temperature, doppler spectrum) in terms of system geometry and configuration. The dielectric properties of terrestrial surfaces are reviewed, and data on the complex dielectric constant of rocks, soils and vegetation are tabulated. Theoretical models for surface response are provided for a variety of surface classes, including the surface of uniform layers, the surface with large scale undulation, the slightly rough surface, the surface of individual scatterers (Lommel-Seeliger surface) and the empirical models (Lambert surfaces). Finally, the report includes a large number of typical surface responses at microwave frequencies. Measured bistatic scattering coefficients, radar return, and surface brightness temperature for many terrestrial surface classes are presented and interpreted in terms of the influence of such parameters as surface roughness, dielectric constant, polarization, frequency, angle of incidence etc. on the response.