My research focuses on understanding the key drivers of effective CD8+ T cell immunity, in particular on how the abundance and quality of antigen-specific T cells in the preimmune repertoire impacts on primary immune responses. More recently, our work has investigated how ageing undermines primary CD8 T cell responses, in part, through direct effects on naïve CD8 T cells that alter their phenotype and decrease their functionality. To understand the molecular basis of these defects, we have assessed functional, metabolic and transcriptional differences across various subsets of naive CD8 T cells from young and aged mice and humans. Understanding characteristics that drive or delimit effective T cell responses permits the optimization or recovery of T cell function via strategies that target these mechanisms.