Scintillation proximity assay (SPA), characterized by its speed, sensitivity, reliability, and the fact that no separation step is required, has become an important technique in high-throughput screening (HTS) for new drugs, and for investigating their biological interactions. The SPA technique now plays a key role in HTS, in that it can be used in many assay formats including radioimmunoassays (RIAs), ligand-receptor binding assays, and enzyme assays. The SPA-based enzyme assay is usually designed in three formats corresponding to different enzymes: signal removal format for hydrolytic enzymes, signal addition format for polymerase and transferase enzymes, and product capture format for antibodies, DNA probes, receptors or other specific binding proteins. The use of SPA in RIAs has been facilitated by new carriers, such as membranes that can be configured in various shapes and sizes, allowing the assay to be performed on samples from many sources including tissue, serum, plasma or cells. This review presents the principles of SPA, discusses supporting materials and quenching effects, as well as detailed examples of the latest advances.