Abstract The key enzymes that catalyze the insertion of proteins into membranes are the Sec translocase and the YidC membrane insertase. Recent insights into the structure and functional intermediates of these enzymes have provided a first molecular glimpse of how they help the newly synthesized proteins to enter the membrane bilayer. In this process, the new proteins undergo a number of specific interactions in the cytoplasm and at the membrane surface before they insert into the bilayer and translocate their external domains across the membrane. The components involved in this pathway recognize each other at the molecular level, forming a route the membrane protein can move along.