Protein transport across the endoplasmic reticulum membrane can occur by two pathways, a co- and a post-translational one. In both cases, polypeptides are first targeted to translocation sites in the membrane by virtue of their signal sequences and then transported across or inserted into the phospholipid bilayer, most likely through a protein-conducting channel. Key components of the translocation apparatus have now been identified and the translocation pathways seem likely to be related to each other but mechanistically distinct. Protein transport across the bacterial inner membrane is both similar to and different from the process in eukaryotes. Other pathways of protein translocation exist that bypass the ones involving classical signal sequences.