Up to now we have been studying the influence of a single factor on the distribution of a disease or of other health-related characteristics. This allowed us to concentrate on the essential ideas. Most diseases are “multi-factorial”, though; many factors exert their influence together. In this book we shall not deal with general methods that have been developed in order to analyze their joint influence; some tools will be mentioned in Chap. 28. The present chapter is devoted to a different problem, namely that of a factor which “confounds” the action of another one. It is of utmost importance.