The field of genetics holds great promise for furthering our understanding of the etiology of drug dependence and for identifying novel targets for treatment. Genetic studies utilizing twins and families have demonstrated a considerable role for genetics in nicotine and/or alcohol dependence. Risk for alcoholism or nicotine dependence is likely to be the result of a large number of genes, each contributing a small fraction of the overall risk. While this review will focus on studies in humans, many of the candidate genes for human nicotine and alcohol dependence listed here were originally postulated to be important, based on data from animal studies. The review will briefly summarize the results from twin and adoption studies that provide estimations of heritability, the results from chromosomal linkage studies that identify regions of chromosomes that may contain relevant genes, and the results of candidate gene studies. For each topic the data will be presented for nicotine dependence, alcohol dependence, and for nicotine and alcohol dependence together. In addition, each section will review briefly some of the confounding issues in the specific type of approach utilized.