Humans have long used plant and animal extracts for their medicinal properties but until the end of the 19th century their actions were often explained in a speculative manner. The systematic study of drugs did not begin until the 1860s, and the concept of receptors as the target of their effects in the body only emerged at the end of the 19th century from the brilliant and independent studies of John Newport Langley (1852-1925) and Paul Ehrlich (1854-1915). This book addresses the people and the key discoveries that led to the development of the receptor concept and its impact on 20th-century medicine: A.J Clark in the 1930s, and later E.J Ariens and R.P Stephenson in the 1950s provided the quantitative basis of drug receptor occupancy theory. R.P Ahlquist's investigations gave rise to the proposal of receptor subtypes in 1948 and facilitated Sir James Black (and subsequently others) to apply the receptor concept to clinical therapy (beta blockers) in the mid 1960s. By the early 1980s, the first (acetylcholine) receptor had been isolated and cloned. Today, we recognize a large and diverse number of physiological receptors which can be delineated into 'receptor super-families' on the basis of their pharmacology and molecular properties. These receptors are now the focus of a multi-national, multi-billion dollar pharmaceutical industry. This book reveals that these successes were by no means foreseeable, since chance, coincidence, competition among scientists, and other factors played important roles in the history of the receptor concept.