Biological membranes form the interface between cells and their environment. Transport of molecules across the membrane barrier is essential for all cells (e.g. for metabolite supply). As a consequence, many (human) diseases, with cystic fibrosis as the best-known example, are caused by mutations in membrane transport proteins. Unfortunately, our understanding of how these proteins carry out their tasks is still rather limited. To date only ~200 unique membrane protein structures have been deposited in the Protein Data Bank. We are interested in determining the 3D structures of bacterial outer membrane proteins by X-ray crystallography. Those structures are then used, in combination with functional data obtained from biochemical experiments, to propose mechanistic models that can be tested experimentally.