In this examination of obsessive-compulsive behaviour case-histories are described, often in the words of the afflicted, and the effectiveness of various treatments is discussed. This pathological state is now regarded as much more common than was thought earlier, with possibly a million cases in Britain. The boy who couldn't stop washing is only one case described. There is also the man who is convinced, every time he drives his car, that he has run someone over and turns back again and again to check the roadside for a body; the woman who, in an effort to ensure her eyebrows are symmetrical, finally plucks out every hair; a man who cannot enter his front door without completing an elaborate ritual; another who checks his stove hundreds of times a day to make sure he really turned off the gas.