beta-Barrel proteins are found in the outer membranes of bacteria, mitochondria and chloroplasts. The presently known sizes range from small eight-stranded to large twenty-two-stranded beta barrels existing as monomers and oligomers. Their functions are as diverse as active ion transport, passive nutrient intake, membrane anchors, membrane-bound enzymes and defense against attack proteins. Of further interest are the folding process, the crystallization, the observed limited structural diversity and the manifold channel engineering options of these beta-barrel proteins.