Adverse drug reactions (ADRs) are a relatively common cause of morbidity and mortality. Many factors can contribute to ADRs, including genetics. The degree to which genetics contributes to ADRs is not entirely clear and varies by drug, as well as the type of ADR. Pharmacogenetics and, more recently, pharmacogenomics have been applied to the field of ADRs for both predictable ADRs and hypersensitivity drug reactions. Evaluations for glucose-6-phosphate dehydrogenase and thiopurine S-methyltransferase are commonplace clinical tests to reduce hematologic problems associated with drugs, such as dapsone and azathioprine, respectively. Numerous pharmacogenetic associations have been discovered for immediate hypersensitivity reactions to β-lactams, aspirin, and nonsteroidal anti-inflammatory drugs; however, the clinical utility of testing for these genetic associations has not been established. In contrast, pharmacogenetic testing forHLA-B*1502before carbamazepine in patients of certain Asian ethnicities and testing forHLA-B*5701before abacavir treatment are recommended. This review will focus on pharmacogenetics and pharmacogenomics and their role in reducing ADRs, especially those caused by drug hypersensitivity reactions.