CD8⁺ T cells play an important role in immunity to viruses. Just how important these cells are is demonstrated by the evolution of viral strategies for blocking the generation or display of peptide–major histocompatibility complex class I complexes on the surfaces of virus-infected cells. Here, we focus on viral interference with antigen presentation; in particular we consider the importance (and difficulty) of establishing the evolutionary significance (that is, the ability to enhance viral transmission) of viral gene products that interfere with antigen presentation in vitro.