The proteomes of mammalian cells are constantly being processed into sets of representative peptides that are loaded onto MHC I molecules for display at the cell surface where they can be seen by CD8+ T cells. This process, referred to as antigen presentation, reveals viruses hiding inside cells to CD8+ T cells, which can then act to fight the infection. Despite the significance of antigen presentation and CD8+ T cell responses, many aspects of these processes remain uncharacterised for large sets of viral antigens. Thus our understanding is derived from studies of single peptides and reasoned speculation. Examples of outstanding issue include: 1) The dynamic range of presentation during infection; 2) the extent to which cell type dictates the abundance and kinetics of antigen presentation; 3) how the abundance of presented peptides relates to that of their source proteins; 4) how well the CD8+ T cell response is matched to the abundance of peptides displayed on MHC I; 5) the fraction of presented peptides that are virus- versus host-derived; 6) the fraction of virus-derived peptides that are immunogenic. To address these gaps, we are using advanced mass spectrometry methods to detect and quantify the presentation of many peptides on mouse cells infected with vaccinia virus (VACV) and combining this with knowledge of CD8+ T cell responses to VACV in mice. Results from this approach will be presented and discussed in terms of our current understanding of the relationship between antigen presentation and CD8+ T cell resonses, and their ramifications for anti-viral immunity and vaccine design.