Although the decision to continue or to stop prematurely a clinical trial is not solely based on statistical tests, they bring useful objective arguments to the data monitoring board. However, the multiple use of statistical tests leads to increase the risk of false positive conclusions in favor of one of the treatments, and several methods have been developed to address this problem. This article presents the four major strategies that are being used for monitoring clinical trials, as well as the rationale for planning and using such statistical monitoring procedures.