A brief description of membrane bioreactor (MBR) historical evolution has been presented with emphasis on continual decline of treatment costs and energy requirements. Although MBR can operate at biomass (MLSS) concentrations 5 to 10 times higher than activated sludge these concentrations are limited in practice by increasing biomass suspension viscosity that in turn increases “reversible” membrane fouling and decreases oxygen transfer rates. “Irreversible” fouling is a major operational challenge since it depends on subtle interactions of membranes with various fractions of soluble microbial products resulting from microbial metabolism.