Abstract Pharmacogenetics, the study of genetic differences that affect an individual's response to drugs, has outstanding potential to maximize benefit and minimize harm associated with drug therapy. Our rapidly increasing knowledge of all genes in the human genome, together with the development of multiparallel, next generation sequencing technology, is going to facilitate the rapid discovery of genetic variants that lead to reduced drug efficacy, variable dose requirements, and increased risk of toxicity. Implementation of genetic testing into clinical practice has been rather slow, but over the last decade, pharmacogenetics has become increasingly significant. In this chapter we will discuss several clinical areas in which there is evidence of improved patient outcomes with pretreatment genetic testing. We will focus on genetic markers that are already required before drug prescribing or recommended by the regulators.