The papers in ths Special Issue, with their emphasis on physical properties and hypothesis testing, illustrate a growing trend in the field of remote sensing. It is now becoming more common to approach problems by defining the types of data that will be acquired and the analysis algorithms that will be used, based on the geologic setting of the area, the known physical properties (largely spectral at this time) of the materials present and the instrument characteristics. New instruments are currently being developed and tested to acquire multispectral data from aircraft.-from Author