An independent data safety monitoring committee (IDMC) that is established to monitor the safety of participants in a clinical trial has, as its primary purpose, the protection of participants in the trial and the maintenance of the integrity of the trial. The IDMC should operate under a clear charter, with expectations understood by all its members. The investigators and sponsors should trust their IDMCs and give the IDMC the tools that it needs to operate effectively. The need to separate the investigators of randomized clinical trials from ongoing data has become widely appreciated. Especially when the study in question is a so-called 'phase III', or confirmatory, trial, knowledge of ongoing results can subtly or explicitly affect the care of participants in the trial, the methods of data collection, and the recruitment of new participants into the trial. These changes, if data-driven, can induce bias and therefore affect the scientific integrity of the trial. Failure to monitor the ongoing data, however, may put patients at undesirable risk, especially if emerging data indicate an unknown adverse consequence of therapy. The use of IDMCs has increased perhaps in response to the growing sophistication of the community of clinical trialists, the US Food and Drug Administration (FDA) draft guidance on data monitoring committees, the US National Institutes of Health (NIH)'s current rules for data monitoring plans, and the Data Monitoring Committees: Lessons, Ethics, Statistics Study Group from the UK National Health Service (DAMOCLES) recommendations.This article summarizes and describes the design and operations of IDMCs in oncology.IDMCs in oncology share many similarities with IDMCs in other disease areas, but the particular nature of cancer patients affects the operation of these committees.IDMC are important in oncology clinical trials because of the severity of illness and the often toxic effects of treatments.