Compulsive cleaning is an attempt to remove feelings of contamination that threaten one's physical health, mental health or ability to function socially. The fear of becoming contaminated can be complex, powerful, persistent and easily spread. Contamination is defined, the main types of contaminants set out and the characteristics of the fear are described. The distinction between normal and abnormal feelings of contamination is considered, and abnormal beliefs about contamination are analysed. Attention is drawn to the fact that contamination can occur without any physical contact, and the concept of mental pollution is used to elucidate this process. The causes and consequences of contamination fears are described, and some connections between fear and disgust are considered. The concept of cognitive co-morbidity is applied to an analysis of associations between the fear of contamination and obsessions, social fears and phobias. It is suggested that applying cognitive analyses and tactics may improve our ability to treat these powerful and tenacious fears.