Peptide epitopes presented by HLA class I are supplied by an elaborate system of antigen processing which subjects candidate epitopes to a process of selection according to little-understood rules. Transport of peptides from the cytosol into the endoplasmic reticulum by the TAP (transporter associated with antigen processing) complex is likely to play an important role in peptide selection for presentation. The recent development of an assay measuring substrate binding to the TAP complex has led to the identification of the major structural properties of peptides selected by the human transporter. Human TAP favors transport of peptides with structural features common to HLA class I ligands. However, TAP dislikes peptide ligands preferred by some HLA class I alleles, which may therefore frequently rely on alternative sources of peptide ligands.