This brief gives rise to three main themes. First, the book describes some of the reasons underlying the adoption of the conditioning paradigm and the epistemological advantages of the paradigm for behaviour modification. Second, a number of chapters discuss the current trends in specific areas of applied psychology where conditioning principles play an important heuristic role. These chapters deal with the uses made of conditioning theory in the areas of mental handicap, psychiatric therapy, work organizations, and the treatment of brain injury. Third, later chapters discuss some of the more recent theoretical developments in the field of behaviour modification/therapy, in particular the drift from strict behaviouristic applications of conditioning principles to more cognitive ones.