Suggestion, a form of treatment considered outmoded around the turn of the century, now plays a considerable role in general medicine and all contemporary psychotherapies—including psychoanalysis, behavior therapy, and group encounters. The dramatic return of suggestion is seen in the community mental health movement, in which centers dispense brief therapy, help, and advice to large portions of the population previously unreachable. In spite of the ubiquity of suggestion and the large literature that developed at the end of the nineteenth and the beginning of the twentieth century, very little is actually known as to how it 'works', if and when it does. Considerably more research is needed on this subject which stands out as a basic riddle of the behavioral sciences.