The past 20 years have heralded fascinating developments in the field of CTLA-4 biology. The CTLA-4 protein is a critical negative regulator of T cell immunity and its absence provokes severe lymphoproliferative disease. In a surprising twist, the generation of mixed bone marrow chimeric mice revealed that CTLA-4 predominantly functions in a cell-extrinsic manner, suggesting that CTLA-4 expressed on one cell can modify the behaviour of another cell. This was followed by the demonstration that CTLA-4 is highly expressed in regulatory T cells and can contribute to their suppressive activity. In line with a cell-extrinsic function, increasing evidence indicates that CTLA-4-positive cells can modify the phenotype of antigen presenting cells (APC), thereby regulating the priming of naive T cells. Notably, CTLA-4 is able to down regulate expression of costimulatory ligands on APC via a process of trans-endocytosis. The identification of patients with mutations in the ctla4 gene has provided an opportunity to study the contribution of CTLA-4 to Treg function and immune regulation in the human immune system. Finally. it has become apparent that CTLA-4 also plays a role in controlling humoral immunity, via the regulation of CD28-driven follicular helper T cell differentiation. At the recent German Society for Immunology congress, I discussed some of the contributions of my own lab to the unfolding of the CTLA-4 story, in the context of the work of others in the field. Despite the enormous clinical potential associated with modulation of the CTLA-4 pathway, including the use of soluble CTLA-4 molecules in autoimmune settings and blocking antibodies in cancer, it is clear there is still much to learn about this important pathway. (C) 2017 European Federation of Immunological Societies. Published by Elsevier B.V. All rights reserved.