Adaptive clinical trial designs may or may not increase efficiency and success rates, reduce costs and shorten the time for a drug development program so it is very important to fully understand the design before launching a trial. For complicated trial designs like combination with several adaptive features, it is quite common that there is no analytic approach to evaluate the design and its operating characteristics. In that case, trial design simulation will play a key role for sponsors and the regulatory agency in understanding the trial design and determining whether or not the design is adequate. In this presentation, I will talk about the experience in reviewing clinical trial protocols with adaptive designs using simulation as the sole tool to evaluate the design and the challenges we are facing.