When a person develops symptoms of depression, he may seek help from a psychotherapist. But there are instances, not rare, when psychological symptoms arc actually produced by physical diseases or by medications. If not properly diagnosed, correct treatment will be delayed. This lecture takes up the general nature and scope of this problem, names criteria for making the distinction, and then discusses specific diseases that can cause symptoms of depression, anxiety, and psychosis. A number of case histories will dramatize the difficulties in diagnosis.