Pharmacogenetic is broadly understood as study or clinical testing of genetic variations that contribute to differing response to drugs. In cancer treatment, applications of pharmacogenetic cover three areas: avoidance of adverse drug reaction (ADR), selection of treatment options, and prediction of cancer recurrence. Patients with genetic variations in UGT1A1 and DPYD genes are hypersensitive to Irinotecan and 5-Fluorouracil (5FU) respectively. Therefore, the chance for the patients to suffer from ADR from using those drugs can be predicted a priori by simple genetic tests. Secondly, the efficacy of targeted therapy drugs such as Cetuximab and Erlotinib, or non- targeted agents such as temozolomide and nitrosourea has been influenced by the presence of certain genetic or epigenetic markers in tumors. Lastly, microarray analysis to evaluate 70-gene expression profile in breast cancer samples has been shown in recent studies to predict probability of breast cancer recurrence. Patients whose tumors have been determined to have low probability score based on the gene expression profile may omit chemotherapy altogether, avoiding unnecessary therapeutic side effects. In summary, pharmacogenetic tests help patients, their caregivers, and doctors in deciding the best treatment options with favorable chance of success, as well as saving overall treatment costs.