Clinical phase I studies in Hematology and Oncology are characterized by specific ethical and methodological considerations. Among these, the determination of a safe starting dose, the definition of dose escalation schemes and the strategies of cohort size expansion per dose level and toxicities observed are of crucial importance. This review will focus on the continual reassessment method, a novel Bayesian-based approach to the conceptual design of clinical phase I studies in Hematology and Oncology. The original method as described by O’Quigley et al. [Biometrics 1990;46:33-48] will be reviewed as well as modifications which are intended to increase patient safety. Finally, clinical applications of the modified Continual Reassessment Method will be discusse