FOXP3+ regulatory T cells (Treg) control T cell activation and effector functions and are key players in immune homeostasis and self-tolerance. Given their central role in preventing autoimmune responses, Treg are considered important targets for the treatment of autoimmune inflammation and several strategies are being explored to enhance Treg numbers or function for the treatment of autoimmune disease. In addition, numerous research groups have studied the presence, phenotype and function of Treg in patients with autoimmune disease. Whether deficiencies in Treg underlie human autoimmune pathology, however, is still a subject of debate. In this thesis we investigated Treg numbers and function in patients with juvenile idiopathic arthritis (JIA), one of the most common autoimmune diseases in children, characterized by chronic inflammation of the joints. In these patients Treg from the site of inflammation can be studied, because during treatment synovial fluid is taken from inflamed joints from which immune cells can be isolated. With this approach we established that Treg are not deficient in JIA patients and are even enriched at the site of autoimmune inflammation. Furthermore, Treg from inflamed joints display efficient suppressive capacity. Still, effector T cells (Teff) from the site of inflammation are insufficiently controlled, because these cells are resistant to suppression. This unresponsiveness to regulation has now been shown for Teff from patients with other autoimmune diseases as well, including type 1 diabetes and inflammatory bowel disease, indicating that it is a general mechanism underlying autoimmune pathology. As a consequence, Teff resistance to suppression might be an attractive target to treat autoimmune inflammation. However, to effectively target unresponsiveness to suppression the underlying mechanism needs to be clarified. Our experiments using JIA patient material demonstrate that Teff resistance to suppression is, at least partially, caused by PKB/c-akt hyperactivation. Teff from inflamed joints display enhanced PKB/c-akt activation and treatment with a specific PKB/c-akt inhibitor restores responsiveness of the cells to suppression. Pro-inflammatory cytokines TNFa and IL-6, which are highly present in inflamed joints, are involved in inducing this PKB/c-akt activation and subsequent resistance to suppression. As a result, inhibition of TNFα, and to lesser extend IL-6, reduces PKB/c-akt activation in inflammatory Teff and enhances their responsiveness to suppression. In addition, we show that pro-inflammatory monocytes present at the site of autoimmune inflammation contribute to impaired T cell regulation as well. The data described in this thesis indicate that targeting Treg for the treatment of autoimmune disease might be less efficient than initially anticipated. Instead, restoring responsiveness of Teff to suppression, either directly, by targeting PKB/c-akt, or indirectly, by interfering with the pro-inflammatory environment, might hold greater promise. However, to permanently restore immune balance in patients with autoimmune disease, in other words to cure disease, a multifactorial approach might be required. Future research should focus on the critical inflammatory mediators that need to be targeted in order to establish long-lasting disease remission, taking in to account the factors interfering with T cell regulation described in this thesis.