In the last two decades T cell function has been analyzed in vitro from many different angles with a great deal of attention dedicated to the basic requirements of activation. During this time a compendium of information has been collected and has proven to be invaluable. Paradoxically very little is known about T cell activation and function in vivo. In the last decade a number of models have been developed which allow the tracking of Ag-activated T cells in vivo and these studies have been instrumental in advancing the field of T cell biology. In particular, a new and emerging paradigm of T cell immunity is evolving.