Jack Siemiatycki is an epidemiologist. He has contributed to several areas of health research, most notably cancer epidemiology. He developed novel and influential methods of exposure assessment to investigate environmental and occupational causes of cancer, and has carried out large population-based studies of several types of cancer. His are among the most frequently cited epidemiologic results in the authoritative Monographs on Evaluation of Carcinogens of the International Agency for Research on Cancer. He has contributed to development of epidemiologic methodology. He has more than 300 publications and 150 invited presentations, and he has been an invited expert on over 100 Boards, Advisory Councils and Expert Panels. He has participated as an expert witness in some high-profile legal litigations, including the most important class action against the tobacco industry in Canada.