Our understanding of anticancer pharmacodynamics, and the relationships between pharmacologic measurements and clinical effects, has grown markedly in recent years due to advances in analytical and computational technology. Although methotrexate plasma levels have been empirically used to guide leucovorin dosing during high-dose methotrexate therapy, there has been no other standard use of therapeutic drug monitoring in oncology. More recently, investigators have attempted to titrate precisely the dose of antineoplastic agents based on previously derived models and real-time analysis of plasma drug or tissue concentrations. Studies have been completed or are in progress using hexamethylene bisacetamide, etoposide, teniposide, fluorouracil (FUra), and cytarabine (ara-C). Future studies will focus on optimal sampling strategies, analysis of intermediate biochemical end points, combination chemotherapy, modulation by colony-stimulating factors, and more sophisticated pharmacodynamic models.