Molecular epidemiology is the study of the distribution and determinants of disease in human populations using techniques of molecular biology and epidemiology. Molecular epidemiology has found many domains of application, in particular cancer epidemiology, environmental epidemiology and infectious disease epidemiology. During the last two decades, molecular epidemiology has become an important discipline in cancer research. The early contributions of molecular epidemiology came from the application of markers of exposure, such as measurements of adducts in blood and urine, in population-based studies. The investigation of aflatoxin exposure markers in cohorts at high risk of liver cancer remains a paradigm of the potential of the new approaches. Over the years, molecular cancer epidemiology has evolved towards the development, validation and application of markers of susceptibility and, more recently, markers of mechanisms of cancer development. This IARC Scientific Publication originates from the workshop ‘Mechanistic Considerations in the Molecular Epidemiology of Cancer’ jointly organised by IARC and the University of Vermont and held in Lyon, France, in November 2001. The workshop was devoted to providing guidelines for use of molecular techniques in cancer epidemiology, in particular with respect to the study of cancer mechanisms. Molecular cancer epidemiology is often defined in terms of biomarkers, which are found internally within biological systems as indicators of exposure, effects, or susceptibility is no longer necessary. For example, DNA adducts are markers that integrate exposure, effect and susceptibility. The new IARC Scientific Publication covers from an interdisciplinary perspective the contribution of molecular epidemiology to the understanding of mechanisms of carcinogenesis, and represents a further contribution to the development of molecular epidemiology as a mature scientific discipline.