Rac1, a member of the Rho family of GTPases, is an intracellular transducer known to regulate multiple signaling pathways that control cytoskeleton organization, transcription, and cell proliferation. Deregulated expression or activation patterns of Rac1 can result in aberrant cell signaling and numerous pathological conditions. Here, we highlight the physiological functions and signaling mechanisms of Rac1 and their relevance to disease.