Theories of instinctive behavior proposed by Lorenz and Tinbergen have been analyzed with respect to long-standing criticisms leveled at them, as well as to neurophysiological evidence. A theory of instinct that meets the criticisms and reconciles the ethological and neurophysiological evidence is proposed. This theory retains the fruitful concept of the Innate Releasing Mechanism introduced by Lorenz and unifies the description of instinctive behavior in terms of hierarchies of feedback loops organized as Behavior Units. The latter are activated by a complex organization of IRM's and Selection Units operating continuously among a host of competing pressures, internal and external, to select the most appropriate action. Various assumptions about the role of learning in instinct-dominated animals are exposed and examples are given of the connection between our assumptions about learning and the modified theory of instinct. To validate the theory and permit its development in greater detail, a computer simulation of the theory has been undertaken, using an artificial animal, ADROIT, as the “guinea pig.” Progress to date with this simulation is discussed.