Why do some people become obsessed with cleanliness, fear of causing accidents, or the idea that they are guilty of some fault or imperfection? Where should the line be drawn between "normal" obsessions, from which everyone suffers to a greater or lesser degree, and pathological obsessions? When should measures be taken to treat those who suffer from obsessions? Why have obsessive-compulsive disorders become so common (2.5% of the population now suffer from them)? Jean Cottrauxs study of several clinical cases enables him to describe how obsessive-thought processes function. Doctor Jean Cottraux is a clinical psychiatrist and lecturer at the Université de Lyon I.