Resistance to chemotherapy is a major cause of mortality in patients receiving treatment for most types of cancer, and overcoming drug resistance has become an important focus of current research. A major clinical challenge is the fact that most anticancer drugs have a narrow therapeutic range, that is, their effective dose is relatively close to that associated with substantial toxicity. Significant advances have been achieved in event-free survival of patients with many types of cancer (most dramatically childhood acute lymphoblastic leukemia, ALL) through a better understanding of the pathobiology of human cancers, the cellular mechanisms of cancer chemotherapy, and the determinants of inter-individual differences in drug effects and treatment response. It is anticipated that expanding our knowledge of these areas will lead to the development of new anticancer agents and to more effective use of existing cancer chemotherapy. Pharmacogenomics research aims to elucidate the genetics determinants of drug efficacy and toxicity. Results of recent studies indicate that both qualitative and quantitative genomic analyses may be required for precise pharmacogenomic characterization of some types of human cancer.