Substance (e.g., heroin) dependence has been influenced by both genetic and environmental risk factors. It has been estimated that genetic factors contribute to 40%–60% of the vulnerability to drug addiction, and environmental factors provide the remainder. The aim of the work in this book is to identify genetic markers within selected candidate genes that may influence substance dependence susceptibility and treatment responses. Pharmacogenetic studies can assess the effects of genetic variation on the risk for a particular phenotype (e.g., being an alcoholic). In addition, pharmacogenetic variability in treatment efficacy and adverse reactions can be investigated to identify particular genetic variants associated with altered responses. Knowing the genetic factors that are involved in substance dependence and treatment will allow us to more accurately match individuals to different treatment options. Our improved understanding of the role of genetics in drug metabolism may also provide additional information about an individuals’ metabolic pattern or drug consumption patterns. The information may also enable forensic toxicologists better interpret post-mortem toxicology results.