The transporter associated with antigen processing (TAP), a member of the family of ABC transporters, plays a crucial role in the processing and presentation of the major histocompatibility complex (MHC) class I restricted antigens. TAP transports peptides from the cytosol into the endoplasmic reticulum, thereby selecting peptides matching in length and sequence to respective MHC class I molecules. Upon loading on MHC class I molecules, the trimeric MHC class I/beta2-microglobulin/ peptide complex is then transported to the cell surface and presented to CD8+ cytotoxic T cells. Abnormalities in MHC class I surface expression have been found in a number of different malignancies, including tumors of distinct histology, viral infections, and autoimmune diseases, and therefore represent an important mechanism of malignant or virus-infected cells to escape proper immune response. In many cases, this downregulation has been attributed to impaired TAP expression, which could be due to structural alterations or dysregulation. This review summarizes the physiology and pathophysiology of TAP, thereby focusing on its function in immune responses and its role in human diseases.