Cytokines have long been known to profoundly influence the adaptive immune response by determining CD4 T cell differentiation. Although IL-6 has been initially characterized as a B cell growth factor and inducer of antibody production research from our lab and others has revealed over the last years that IL-6 also plays a significant role in CD4 T cell differentiation. This review highlights the variety of ways in which IL-6 affects CD4 effector functions and how this may contribute to different types of diseases.