Cytotoxic T lymphocytes (CTLs) are critical for defense against intracellular pathogens and cancer. CTL differentiation begins when a naïve CD8+ T cell recognizes a peptide-MHC ligand complex through its T cell receptor (TCR), triggering a cascade of activation events. The interaction of the TCR with a particular peptide-MHC complex is exquisitely sensitive, such that single amino acid changes in the presented peptide can dramatically alter the activation of responding T cells. Multiple mechanisms have been proposed to underlie this phenomenon, but these cannot be distinguished through snapshot measurements in bulk cellular populations. In this talk, I will describe how we address this using multidimensional measurements (single-cell RNAseq and mass cytometry) of molecular activation events in individual cells over time.