Simple phobias have been one of the most important anxiety disorders in terms of the development of cognitive-behaviour therapy. However, recent developments in conditioning theory and cognitive psychology have led to important changes in the conceptualization of phobic disorders. These developments are reviewed and their implications for a cognitive-behavioural assessment and treatment of simple phobias are outlined, his argued that a phobia is an instance of an atavistic fear which may become phobic as the result of (a) a general stressor, (b) conditioning, (c) vicarious transmission, or (d) verbal transmission. Each path modifies performance as the person acquires the belief that certain situations are associated with specific dangers. Following assessment of the presumed aetiology a formulation of phobias is presented which leads to a rationale for treatment. The treatment of choice is argued to be exposure to the feared situation. The mechanism of treatment is discussed in terms of modern conditioning theory and compatibility with cognitive therapies is outlined