Today, the initiation of any medical treatment still raises questions about its efficacy and safety. Indeed, therapeutic responses vary over time and between individuals and are influenced by age, sex, other treatments and the pathology itself. Genetic factors are thought to be responsible for 20 to 95% of these individual variations. Recent advances in biotechnology, molecular genetics and genomics allow a better understanding of drug metabolism and action. Pharmacogenetics, compiling phenotypic and genotypic data, may lead to a more personalized treatment. To allow a better approach of pharmacogenetics and pharmacogenomics, we will define these two terms and describe their actual and future clinical application.