The transporter associated with antigen processing (TAP) is essential for peptide delivery from the cytosol into the lumen of the endoplasmic reticulum (ER), where these peptides are loaded on major histocompatibility complex (MHC) I molecules. Loaded MHC I leave the ER and display their antigenic cargo on the cell surface to cytotoxic T cells. Subsequently, virus-infected or malignantly transformed cells can be eliminated. Here we discuss the structure, function, and mechanism of TAP as a central part of the peptide-loading complex. Furthermore, aspects of virus and tumor escape strategies are presented.