The IUPHAR/BPS Guide to PHARMACOLOGY is an open-access website, acting as a portal to information on the biological targets of licensed drugs and other small molecules. The Guide to PHARMACOLOGY (with GtoPdb being the standard abbreviation) is developed as a joint venture between the International Union of Basic and Clinical Pharmacology (IUPHAR) and the British Pharmacological Society (BPS). This replaces and expands upon the original 2009 IUPHAR Database (standard abbreviation IUPHAR-DB). The Guide to PHARMACOLOGY aims to provide a concise overview of all pharmacological targets, accessible to all members of the scientific and clinical communities and the interested public, with links to details on a selected set of targets.  The information featured includes pharmacological data, target, and gene nomenclature, as well as curated chemical information for ligands. Overviews and commentaries on each target family are included, with links to key references.