Thalidomide is an immunomodulatory and antiangiogenic drug. Although the exact mechanism of action is not fully understood, it has been shown to be active in a variety of diseases. There are multiple trials going on to evaluate the optimal dose of thalidomide and the importance of combining thalidomide with other drugs. This review introduces the properties and putative mechanism of action of thalidomide and summarizes the most important clinical trials with this biological modifier.