Data Safety and Monitoring Boards were introduced as a mechanism for monitoring interim data in clinical trials as a way to ensure the safety of participating subjects. Procedures for and experience with DSMBs has expanded considerably over recent years and they are now required by the NIH for almost any interventional and for some observational trials. A DSMB's primary role is to evaluate adverse events and to determine the relationship of the adverse event to the therapy (or device). Interim analyses and early termination of studies are two aspects of DSMBs that are particularly difficult. This chapter will discuss the role of DSMBs and address the aforementioned issues. © Springer Science + Business Media B.V. 2008.