Much progress has been made in treating human malignancies and there are now multiple treatment options with similar efficacy for nearly every type of cancer. However, the narrow therapeutic index of most chemotherapeutic agents and the severe consequences of undertreatment or overdosing have led to research molecular predictive factors of the toxicity and efficacy of cancer treatments. Genetic factors affecting drug metabolism and transport partly explain interindividual variability in drug response. Pharmacogenetic focuses on the molecular mechanisms involved in drug response, and its ultimate goal is the optimisation of the treatments, that combines the optimal efficacy and the minimal risk of severe side effects. Polymorphisms in genes encoding specific drug-metabolising enzymes can result in individuals in the general population being characterised as low, rapid or even ultra-rapid metabolisers. Phenotyping and genotyping tests are now available that determine or predict the metabolic status of an individual and, thus, enable the evaluation of risk of drug failure or toxicity. Some clinical applications of pharmacogenetics (5-FU, irinotecan, thiopurines) have already been developed in routine medicine resulting in significant improvement in patient treatment. The clinical validation of an increasing number of pharmacogenetic tests, as well as the development of new highly efficient technologies for genotyping (real-time PCR, DNA chips...) should further promote pharmacogenetics in clinical practice and lead to the development of a patient-tailored drug therapy.