We are interested in the signaling complexes that control T cell differentiation and activation. Using advanced imaging techniques we examine the assembly and localization of these complexes in living T cells. The composition of these structures varies dynamically in a fashion that reflects the physiological constraints on T cell activation. We have shown that these structures cannot persist in the absence of any of several critical proteins. These proteins are therefore likely to form a cooperatively assembled core complex upon which complete signaling complexes are built. Finally, we have found that these complexes change in localization over time. We are currently defining the components of the core complex and are testing how changes in localization affect the generation of signals uniquely required for T cell activation.