Ligand binding to receptors is a key step in the regulation of cellular function by neurotransmitters, hormones, and many drugs. Not surprisingly then, genome projects have found that families of receptor genes form the largest groups of functional genes in mammalian genomes. A large body of experimental data have thus been generated on receptor-ligand interactions, and in turn, numerous computational tools for the in silico prediction of receptor-ligand interactions have been developed. Websites containing ligand binding data and tools to assess and manipulate such data are available in the public domain. Such Websites provide a resource for experimentalists studying receptor binding and for scientists interested in utilizing large data sets for other purposes, which include modeling structure-function relationships, defining patterns of interactions of drugs with different receptors, and computational comparisons among receptors. The Websites include databases of receptor protein and nucleotide sequences for particular classes of receptors (such as G-protein-coupled receptors and nuclear receptors) and of experimental results from receptor-ligand binding assays, as well as computational tools for modeling the interactions between ligands and receptors and predicting the function of orphan receptors. In this chapter, we provide information and Uniform Resource Locators (URLs) for Websites that facilitate computational and experimental studies of receptor-ligand interactions. This list will be periodically updated at https://sites.google.com/site/receptorligandbinding/.