Genetic epidemiology focuses on the familial and, in particular, genetic, determinants of disease and the joint effects of genes and environmental factors. Thus, genetic epidemiology aims at quantifying the risk of disease, cancer included, associated with genetic variation within a given population. In spite of the fact that most cases of cancer are initiated based on environmental exposures accumulated during a life-time, the carcinogenetic process itself is governed by a set of specified and unspecified genetic malfunctions, making genetic epidemiology a valuable methodological platform in cancer research. Details are given on known and suspected causes of cancer, including presumable gene-gene and gene-environment interactions.