Immune cells, and T cells in particular, display an enormous degree of developmental plasticity, which manifests itself in the existence of multiple functional subsets, each displaying characteristic effector functions, cytokine profiles and tissue specificities. The SFB/CRC 1054 has set out to explore the determinants of this remarkable plasticity of cell-fate decisions in the immune system, both in terms of the developmental decisions of precursor cells under homeostatic conditions and in terms of the flexible and often reversible adaptation of ‘mature’ cells to immune challenges.