P450 prodrug activation-based cancer gene therapy strategies have been developed and show striking effectiveness in both in vitro cell culture and preclinical antitumor animal models (1 ,2 ). In vivo tumor models play an important role in the evaluation of the therapeutic efficacy of these strategies, and complement in vitro approaches (see Chapter 7) designed to evaluate and compare different P450 genes and different P450 gene/prodrug combinations. This chapter describes methods required for in vivo tumor models and their use in evaluating the therapeutic impact of intratumoral P450 gene expression on a tumor’s chemosensitivity to cancer chemotherapeutic agents.