T cells display many different phenotypes and functions, depending on the nature of previously encountered signals. If we want to understand how these different T cell subsets arise, we need to be able to follow individual T cells and their progeny through time. With the aim to map the life histories of individual T cells we have developed unique technologies that allow us to determine whether different T cell populations arise from common or distinct progenitors.Within this project we will utilize genetic reporter systems to determine:1. How T cell recruitment, proliferation and death shape antigen-specific T cell responses2. At which stage the resulting T cells commit to the effector or the memory T cell lineage3. The self renewal potential of the tissue-resident memory T cells that remain after infection is clearedBy following T cells and their progeny through time, this project will describe the regulation of cell fate in antigen-specific T cell responses. Furthermore, this project will lead to the creation of novel reporters of cellular history that will be of broad value to analyze cell fate and kinship for a variety of cell types.