There have been tremendous advancements in application of modeling and simulation (M&S) in drug development during the last decade. The pharmaceutical companies started to pay more attention to implement simulation exercises in drug development in order to achieve cost effectiveness. The Food and Drug Administration (FDA) published a white paper titled, Critical Path Initiatives, in March 2004. This puts forward model based drug development that calls for use of quantitative M&S to facilitate informed decisions. The European Medicines Agency (EMEA) also encourages use of simulations in guiding drug development. With this much interest, Clinical Trial Simulations will serve as a reference to understand how clinical trial simulations are being used in drug development. Clinical Trial Simulations compiles the topics of recent interest and the case studies of how clinical trial simulations were used in various therapeutic areas. It is divided into parts that describe subjects that have gained interest recently; application of M&S in regulatory decisions; application of M&S in various therapeutic areas; and special use of M&S.