Human Leukocyte antigen (HLA) molecules play a key role in health and disease by presenting antigen to T-lymphocytes for immunosurveillance. Immunopeptidomics involves the study of the collection of peptides presented within the antigen binding groove of HLA molecules. Identifying their nature and diversity is crucial to understanding immunosurveillance especially during infection or for the recognition and potential eradication of tumours. I will discuss new insights into the fidelity of antigen processing and presentation and how novel informatics approaches and databases have shed light on the diversity of peptide antigens derived from unconventional sources including peptides derived from transcripts associated with frame shifts, long noncoding RNA, incorrectly annotated untranslated regions, post translational modifications and proteasomal splicing. Several challenges remain in successful analysis of immunopeptides, yet these developments point to unexplored biology waiting to be unravelled.