A clinical trial is a planned experiment on human subjects to assess one or more potentially beneficial therapies. A central problem in early phase clinical trials is the limited knowledge on the new treatment of interest. As a consequence, extreme caution needs to be taken in study designs to minimize the risk of participants while maximizing the benefit. This chapter provides an overview of the recent advances in statistical designs of early phase clinical trials. Since formal statistical methods for phase I and II trials have been mostly developed for cancer drugs, a considerable portion of this chapter addresses statistical issues in this particular setting.