This book provides an overview of the fundamentals of clinical trials, the key terminologies and concepts, a brief review of basic statistics, and Bayesian and frequentist estimation and inference procedures. There has been enormous interest and development in Bayesian clinical trial designs, especially for early phases of a trial. However, for phase III trials, frequentist methods still play a dominant role based on hypothesis testing and controlling type I and type II errors. This book addresses various statistical methods that are commonly used for designing clinical trials and interim monitoring and analysis from both Bayesian and frequentist perspectives. Comprehensive discussions on a variety of statistical designs, their properties, and operating characteristics for phase I, II and III clinical trials are provided, as well as an introduction on phase IV trials. Many practical issues and challenges arising in the clinical trials are addressed, and while the book mainly focuses on the Bayesian approaches for phase I and II trial designs, many important frequentist methods for phase III clinical trials are included. In addition, advanced and up-to-date topics such as risk analysis of toxicity and efficacy, seamless phase I/II trial designs, phase II/III trial designs, drug-combination trials, targeted therapy designs, adaptive randomization, and causal inference are introduced. Chapter coverage includes Fundamentals of Clinical Trials; Frequentist versus Bayesian Statistics; Phase I, II, III, and IV Clinical Trial Design; Dose-response Curve; Dose Finding in Drug-combination Trials; Adaptive Randomization; Causal Inference and Noncompliance; and Targeted Therapy Design.