Accumulating data may be reviewed regularly in all phases of clinical development for decision-making based on safety or clinical benefit. This chapter discusses the process of reviewing accumulating clinical trial data in a formal manner. It provides an overview of the structure and the operations of a data monitoring committee (DMC), and elucidates the statistical issues and challenges of interim monitoring. The chapter describes several commonly used approaches for interim monitoring. Multi-center trials must be coordinated and administered efficiently. A DMC's objectivity is in part due to independence of the members. A formal statistical framework can enhance the objectivity by providing a universal language to communicate the accumulating evidence. Group sequential methods for determining the critical values to be used during interim analyses represent a key advancement in the theory and application of sequential analyses. EAST is highly regarded as an excellent tool for interim monitoring of clinical trials.