Behavioural psychotherapy has long historical roots. Recently it has led to effective treatment for selected neuroses, including phobic, obsessive-compulsive and sexual disorders. Potent therapy has become a tool of experimental psychopathology which advances theory and practice. A pervasive principle is exposure of the patient to those stimuli which evoke his discomfort until this subsides. Level of arousal during exposure does not affect outcome. Theoretical issues are reviewed which decide when exposure will be sensitizing or habituating. Both psychoanalytical and conditioning models of neurosis are out of date, and models derived more directly from clinical experiment are becoming possible. The aetiology of phobias and rituals can be seen as failed extinction rather than enhanced acquisition. Relevant phylogenetic and biological factors are discussed. At the other extreme, well-documented faith-healing indicates huge gaps in our knowledge of psychotherapy.