The combination of the words operant and conditioning have come to characterise an area of research in psychology which owes much to the pioneering theoretical and experimental work of B. F. Skinner. The typical operant conditioning experiment specifies the form of behaviour which is to be associated with a reinforcer, rather than allowing the reinforcer to have coincidental relationships with any pattern of behaviour. The nature of the reinforcer studied in operant conditioning experiments is also somewhat arbitrary, usually being food or water for suitably deprived subjects. The experimental arrangements themselves are usually automated, and this makes it possible to investigate complex interrelationships between operant behaviour and environmental events. The most important reason why operant conditioning experiments are reported with extensive cumulative records is because each record is an uncontaminated account of an individual animal's behaviour over relatively long periods of time.