In the second half of the twentieth century, epidemiology came to shape public health discourses and practices to an unprecedented extent. The chapter explores the transformation of the discipline after World War Two and analyses the crucial debate on the notion of "causation" that sprung from the growing interest in non-transmissible, chronic diseases. A landmark in this history was the controversy over the interpretation of the statistical relationship between smoking and lung cancer prompted by American and British publications in 1950. This sometimes heated debate also provided Austin Bradford Hill with the opportunity to set out his "pragmatics" of risk factor epidemiology.