Resonance energy transfer is widely used in studies of biomolecular structure and dynamics. It provides information about distances on the order of 10 to 100 A and is thus suitable for investigating spatial relationships of interest in biochemistry. The information available from energy transfer studies has been enhanced by the advances in instrumental methods and procedures of data analysis related to fluorescence decay studies. Some practical aspects of the method are reviewed. These include sample preparation, Förster type distance determination, methods of detecting energy transfer and calculating transfer efficiency, time-resolved measurements, and data analysis. Applications of resonance energy transfer, including qualitative measurements as well as microscopy, average distance estimation, and distance distribution analysis, are surveyed.