The use of peptides to infer protein expression is very familiar to all of those who undertake bottom-up proteomics studies of cells, tissues, and fluids. Remarkably, an analogous process is used by the immune system to report on the presence of infectious microorganisms, malignancy, and other abnormalities within the body. This process known as antigen processing and presentation relies on the selection and presentation of peptides on the surface of cells as a bound complex with major histocompatibility complex (MHC) molecules. The isolation and characterization of these MHC-bound peptides, predominantly through the use of mass spectrometry, has been coined immunopeptidomics and is the subject of this special issue.Immunopeptidomics as a field has rapidly matured in the past decade and has moved from being the domain of just a handful of specialized laboratories to now being more generally applied by the proteomics and immunology communities. The development of immunoproteomics was pioneered by the groups of Don Hunt with Victor Englehard and Alessandro Sette [1][2][3][4][5] and Hans-Georg Rammensee. [6][7][8][9] Over the years, the protocols developed by these groups have been refined but the basic premise of isolating peptide-MHC complexes under near-native conditions and the analysis of the bound peptide cargo primarily by LC-MS/MS remain unchanged. What has changed is the instrumentation and sensitivity of the measurements which have allowed analysis of thousands of peptides using this technique. With such information-rich and deep datasets reporting on peptide epitope display by a variety of different cell types and tissues have come profound insights into fundamental mechanisms of antigen presentation (e.g., refs. 2,4,10-17) and how this may be perturbed by various environmental or infectious agents (e.g., refs. 18-20). The maturation of immunopeptidomics has also facilitated the identification of T cell epitopes in a variety of diseases including autoimmunity (e.g., refs. 21-24), cancer (e.g., refs. 25-31), and infectious disease (e.g., refs. 32-35). It has revealed the critical role that post-translational modification (e.g., refs. 24,30,31,36-40) has in the immune response and allowed enumeration of specific epitopes with great accuracy and precision (e.g., refs. 41-43).In this special issue, we have collected contributions that apply immunopeptidomics in the context of infectious disease, [44,45] cancer, [46] and graft versus host disease. [47] Furthermore, the development of HLA binding prediction and the use of highquality mass spectrometric data are discussed in two review articles. [48,49] Enrichment strategies prior to MS analysis are compared for suitability of antigen discovery [50] and alternative instrument acquisition strategies to the standard data-dependent acquisition (MS3, [45] PRM, [47] EThcD [51] , and DIA [52] ) are discussed. An important outcome of a recent meeting that focused on immunopeptidomics and the establishment of the Human Immunopeptidome project [53] is a series of guide...