Occupation and industry classifications are used in epidemiological studies to classify study subjects according to their job and subsequently to study risk by job, to infer social class indicators, or to infer exposure to specific agents through job-exposure matrices. However, documentation on methodological aspects concerning the use of occupation and industry classifications is sparse within epidemiology. This paper reviews the diverse applications of occupation and industry classifications in population-based epidemiological studies. The different classifications in use are discussed, and criteria are given for choosing a classification in an epidemiological study. Finally, the reliability of coding for occupation and industry is reviewed. A further standardization of the use of occupation and industry classifications in epidemiology is recommended, in order to facilitate future comparisons between studies and fully exploit their possibilities, especially when occupational exposures are to be inferred.