The terms competition and competitive were in use for appropriate types of interaction in human and animal behaviour from the seventeenth century. In the nineteenth and early twentieth centuries they reached more technical uses in biology, especially in darwinian studies; and in chemistry in describing competing reactions, surface phenomena and the influence of substituent groupings in reactant molecules. Use of competitive and non-competitive to describe enzyme inhibitors had a specific beginning when J. B. S. Haldane (following premonitory work of others) applied the terms in 1927 and 1930 to types of inhibition already differentiated by Michaelis and co-workers. The theoretical background in kinetics and stereochemistry so acquired gave a firmness to the application of the terms in biochemistry. The first examples concerned glycosidases, especially beta-D-fructofuranosidase or invertase, and interactions of carbon monoxide and oxygen at iron-porphyrin systems. They were thus of interest in toxicology and in enzyme and carrier studies. The sphere of application of the biochemically-defined terms expanded greatly when, following investigation of sulphonamide action, it was realized that concepts of enzyme inhibition by structurally related compounds offered a route to understanding the action of existing medicaments and to the production of new ones. Ideas and terminology based on competitive and non-competitive enzyme inhibition and receptor occupancy have subsequently been applied in many ways. Examples include application to the analysis of feedback inhibition and other processes of metabolic control; to receptor relationships among neurotransmitters and medicaments; and to understanding interactions at sensory receptors.