Increasingly, clinical trials are conducted using group sequential methods in order to address the ethical and efficiency requirements for performing experiments with human volunteers. The design, conduct, and analysis of a sequential clinical trial is necessarily more involved than that for a clinical trial in which the data would only be analyzed at the end of the study. In this module we provide an introduction to methodology for the design and evaluation of group sequential trials from both a frequentist and Bayesian perspective. Methodology will presented using case studies and examples of clinical trials with continuous, binary, and survival endpoints. The sequential methods will be illustrated using RCTdesign, an R module for the design, monitoring, and analysis of clinical trials. Emphasis will be placed on both the scientific and statistical impact of group sequential trial design.