The field of group psychotherapy has been reviewed with the intention of clarifying its purposes, methods, and mechanisms. We cannot expect consensus at a time when group psychotherapy is still rapidly expanding and no objective integrative concept has evolved. It is inevitable that trying to summarize the extensive literature probably results in more questions raised than answers provided. As with many other therapies we are more certain of possessing an effective method than of knowing the reasons for its effectiveness. True enough, many individual therapists are sure that they know the dynamics with which they operate; but their findings and reports are so divergent and often contradictory, that a scientifically reliable answer seems to elude us. The integration of all available information into one body of knowledge would be a Herculean task; but group psychotherapy may be able to accomplish what psychiatry has been unable to do—integrate the various dynamic theories into one body of scientific knowledge. It is the medium of group psychotherapy itself that may promote such long-overdue integration. The therapy group provides facilities for controlled observation that hardly exist in individual cases. Furthermore, a spirit of increased cooperation characterizes all those who have experienced group psychotherapy and benefited from it, patients and therapists alike. Group therapists seem to be willing to exchange their findings and—regardless of their orientation—discuss their observations in a give-and-take way, essential for scientific progress. In this sense psychiatry may experience a true revolution through the new methods it has invented for the treatment of patients.Abstract Teaser