Metabolic processes often require microcompartmentalization for purposes of efficiency, selectivity and isolation. Many examples are discussed in the articles included in this written symposium. These include sulfur granules that protect the cell cytoplasm from toxic elemental sulfur, and gas vacuoles that provide the function of flotation, particularly important for aquatic photosynthetic organisms. Metabolosomes increase the efficiency of specific chemical reactions and allow the isolation of metabolic processes that could otherwise be detrimental to the cell. Examples include carboxysomes for fixing CO2, proteosomes for degrading denatured or partially denatured proteins, ethanol metabolizing microcompartments, and chaperonins. Protein secretion is often driven by ATPase-dependent machines, and in bacteria many different types of secretory systems are known. The degradation of crystalline cellulose, the most prevalent carbohydrate polymer on Earth, is facilitated by the association of many hydrolytic enzymes in bacterial localized cell surface structures known as cellulosomes. This written symposium provides essential information for biochemists, microbiologists and cell biologists interested in the detailed metabolic functioning of a cell, and for anyone interested in the evolutionary basis for cellular complexity.