The effectiveness of anti-cancer drug therapies is often limited by the difficulty of achieving drug delivery throughout solid tumors. Mathematical models permit an analysis of the factors leading to inadequate drug delivery to tumors and can suggest strategies for improving delivery. An overview is given of key factors that influence drug delivery and the extent to which they have been incorporated into existing theoretical models. These factors include spatial gradients of drug concentration and other variables within tumors and other parts of the body, and the relative magnitudes of the time scales involved in drug transport, tumor cell kinetics, and host toxicity. Models for both systemic and regional delivery methods are considered, including intravenous, intraarterial, intraperitoneal, intrathecal, and intratumoral delivery. Strategies for improving delivery are discussed, including use of two-step therapies, hyperthermia, liposome encapsulation, and magnetic targeting. Until now, modeling has mainly developed in separate subfields of tumor growth and cell kill kinetics, compartmental modeling of the body, spatially distributed models for single tissues, radiation dose calculations, tumor oxygenation, tumor blood flow, and cellular pharmacokinetics. In the future, models that integrate these subfields should be developed.