Receptor binding assay formats for HTS and lead optimization applications are discussed in detail in this chapter. Critical considerations that are discussed include appropriate selection of detection technologies, instrumentation, assay reagents, reaction conditions, and basic concepts in receptor binding analysis as applied to assay development. Sections on special circumstances that address high affinity binders and Hill slope variations are also included and may be useful for data analysis and trouble shooting. A discussion on Scintillation Proximity (SPA), filtration binding and Fluorescence Polarization (FP) assays for receptor binding analysis are also included with detailed accounts on assay development using these technologies.