The internist and the psychiatrist will ever have a common interest in the obscure problems of etiology and pathogenesis of diseases and anomalies that are accompanied by abnormalities of thought, feeling, and behavior. Progress in this direction is bound to be slow for the studies are exceptionally complex and there are many impediments to be removed. Though the problems are deep and difficult, they are doubtless soluble by the mind of man, and they exert an uncommon fascination upon those who visualize them. Causes may be internal or external, and are often a combination of both. The tracing of the direct and indirect relationships between these causes and the abnormal cerebral functioning upon which the disturbances of psychobiological adjustment seem to depend is the task of pathogenesis. The internist who has studied the infantile cerebropathies with their resulting imbecilities, syphilis followed by general paresis, typhoid fever and its toxic delirium, chronic alcoholism with its characteristic psychoses, cerebral thrombosis with its aphasias, agnosias, and apraxias, thalmic syndromes due to vascular lesions with their unilateral pathological feeling-tone, frontal-lobe tumors with joke-making, uncus tumors with hallucinations of taste and smell, lethargic encephalitis with its disturbance of the general consciousness and its psychoneurotic sequelae (lesions in the globus pallidus and their motor consequences), pulmonary tuberculosis with its euphoria, and endocrinopathies like myxoedema and exophthalmic goitre with their pathological mental states, is encouraged to proceed with his clinical-pathological-etiological studies in full assurance that they will steadily contribute to advances in psychiatry. The eclectic psychiatrist who is examining mental symptoms and symptom-complexes ever more critically, who is seeking for parallel disturbances in physiological processes and who considers both psychogenesis and somatogenesis in attempting to account for psychobiological maladjustments will welcome, we can feel sure, any help that internal medicine and general and special pathology can yield. These studies in pathogenesis and etiology are fundamentally necessary for the development of a rational therapy and prophylaxis. Already much that is of applicable value in practice has been achieved. The internist shares with the psychiatrist the desire that knowledge of the facts regarding care, cure, and prevention of mental disorders may become widely disseminated among medical men and at least to some extent among the laity. Experts in psychiatry firmly believe that at least half of the mental disturbances now prevalent could have been prevented, if, during the childhood and adolescence of those afflicted, the facts and principles of existing knowledge and the practical resources now available could have been applied.