An occupational disease may be defined simply as one that is caused, or made worse, by exposure at work. While epidemiological studies of populations can determine whether disease is attributable to a particular type or level of exposure, for an individual patient this is less clear. Judgments about the patterns of exposure likely to be causal may be made in medicolegal cases or claims for compensation but these decisions have little value in determining the true extent of disease caused by work, not least because of the absence of reliable exposure data. Information about the incidence and distribution of such diseases is thus far from complete. This review describes recent advances in the understanding of the patterns and causes of occupational disease.