The urge to master the environment is a powerful motivational force which presents a challenge to the psychoanalytical theory of drives. The author reviews critically the various psychoanalytic interpretations of mastery, beginning with Freud's concept of component instincts, sublimation of the libido, expression of destructive and non destructive aggression, neutral psychic energy and third instinct theory. In the author's opinion none of these gives a satisfactory description of the nature of mastery. The author then examines the development of mastery in children (ontogenesis), beginning with an infant's learning first to exert some control over the infant-caregiver relationship and later to manipulate inanimate objects, while developing perceptual, motor and cognitive skills. Following this the author examines the origins of mastery in early living organisms (phylogenesis). Finally, he presents a hypothesis that seems to offer the best description of the nature of the urge to master, namely, that it is an evolutionary product of the need to exert control over the environment, a precondition to life rather than a characteristic of living matter. Its aim is to assure the availability of essential resources. It is separated from the two basic drives but served by both: aggression directed at obstacles, and narcissism in man and perhaps some higher animals.