MIF has been described as a protein that plays an essential role in both innate and acquired immunity. Previous studies have demonstrated that MIF activates lymphocytes, granulocytes and monocytes/macrophages. Furthermore, MIF can counteract the physiological function of steroids, thus playing a role in immune system regulation. Further evidence for a role of MIF in immunity was obtained in mouse models of autoimmune disorders, where the inhibition of MIF resulted in a more benign disease progression. This observation made MIF an attractive therapeutic target for the treatment of these disorders. Moreover, MIF expression was found to be upregulated in a variety of different tumor cells, a finding that further attracted interest. This review provides an overview of the involvement of MIF in both autoimmune disorders and tumorigenesis and summarizes the molecular action of MIF in this context.