The adaptive immune system plays an essential role in protecting vertebrates against a range of pathogens and cancer. The MHC class I-dependent pathway of antigen presentation represents an elaborate machinery to recognize and eliminate infected or malignantly transformed cells, taking advantage of the proteasomal turnover of the cell’s metastable proteome. The transporter associated with antigen processing (TAP1/2, ABCB2/3) is a crucial component in this pathway, responsible for recognition, translocation and final loading of antigenic peptides onto MHC I complexes in the ER-lumen. These different tasks are coordinated within a dynamic macromolecular translocation complex (PLC) consisting of TAP1/2 as key element, the adapter protein tapasin, the oxidoreductase ERp57, the lectin chaperon calreticulin and the final peptide acceptor the MHC I heavy chain and b2-mircoblobulin. This contributation summarizes the structural organization and molecular mechanism of the antigen translocation machinery as well as various modes of regulation by viral factors as well as in genetic diseases and tumor development.