At a first stage of development, ground truth is gathered in conjunction with radar data in order to establish statistical correlations between observed backscatter and properties of the irradiated surface. Significant correlations have been shown to exist between backscatter and many geophysical quantities; for example scatterometer measurements over the ocean can be used to infer wind speed, synthetic aperture radar (SAR) measurements over woodland can allow the separation of tree species, etc. Once statistical relationships are established, the next stage is to try to understand the physics underlying the relationship, both because of its intrinsic interest and because of the need to optimize system performance for particular applications. This places a second role on ground truth, since it must be used to validate the physical theories stemming from it. Microwave scattering from land surfaces is a topic of growing interest as we look towards land observation missions within the coming decade. Several theories have been proposed to account for scattering from surfaces of a vegetation layer, but none of these has been rigorously tested. If they are to contribute effectively to future land missions, it is essential that such testing be carried out. The first section of this paper describes the requirements such testing will put on ground measurements, and tries to identify whether such measurements are feasible. Microwave scattering mechanisms from the sea surface, and particularly their effects on SAR images of waves, and clutter-statistics in SAR sea-surface images are still a matter of some debate. The second section of this paper outlines the problems inherent in sea imaging with SAR and describes experiments that need to be carried out in order that these questions can be resolved.