Pharmacogenomics (PGx) and pharmacogenetics (PGt) are emerging interdisciplinary areas recently defined by the regulatory authorities at an international level as ‘the investigation of variations of DNA and RNA characteristics as related to drug response’ (PGx), and the study of ‘the influence of variations in DNA sequence on drug efficacy and toxicity’ (PGt). In recent years a number of studies have in fact produced growing evidence that, besides the effects of age, sex, diseases, and different drugs interactions, genetic factors play a role in the inter-individual variability of drugs response. The increasing genomic knowledge has also raised the profile and role of the so called ‘genomic biomarkers’ (GBs) in drug development, approval, and clinical use. The aims of this review are to (a) revisit the general understanding of the role of genomics and genetics in drug response, (b) provide an update on the definition and classification criteria of GBs as recently defined at a global level by regulatory agencies such as the European Medicines Agency and the Food and Drug Administration, and (c) illustrate with some examples current and potential applications of biomarkers in clinical practice and in drug development.