Bacterial membranes have diverse functions, depending on whether they are specialized membranes or cytoplasmic membranes possessing transport, mitochondrial activities and biosynthetic functions for assembly of membranes, walls and capsules. In contrast to plasma membranes which serve as major biochemical organelles of both Gram-positive and Gram-negative bacteria, the outer membranes of the latter group confer barrier functions on the cells, providing a variety of selective channels. Although prokaryotic cells lack the array of membranous organelles characteristic of eukaryotic cells, bacteria with specific physiological and genetic capabilities form specialized membrane systems such as the bacteriorhodopsin purple membrane, chromatophore membranes of phototrophs, and forespore membranes essential to bacterial endospore formation. Unravelling the structure, function and proteins of these membranes presents a formidable biochemical, immunochemical and structural challenge.