Highlights from a Fundación Juan March Interdisciplinary Meeting. CD8⁺ T cells (T_CD8+) play a critical role in immunity to viruses. A central feature of antiviral T_CD8+ responses is immunodominance: out of thousands of potential target peptides, only a handful generate measurable responses. A recent Fundación Juan March Meeting brought together scientists working on the various steps in antigen presentation and T cell biology that contribute to immunodominance, whose understanding is key to rationally developing vaccines meant to elicit effective antiviral T_CD8+ responses.