This QuintilesIMS White Paper provides a comprehensive scientific review of Phase I trials in oncology, and highlights best practices and operational recommendations for their successful delivery. Early-phase studies, which introduce an investigational new drug into human subjects, represent a challenging first step in a new drug’s clinical development program. They are the bridge between basic science and clinical development, and provide pivotal information to select the right drug at the right dose for the right patients to be investigated in the subsequent phases of clinical development.