Sludge quality is fundamentally the most important aspect of successfully operating an activated sludge plant. While much attention can be focused on the final effluent, experienced operators know that reliability producing a high quality effluent depends on the sludge quality of their system. A sludge that flocculates, settles and compacts properly will produce the best effluent a plant is capable of. This paper discusses the microbiological aspects of sludge quality control and examines microbiological phenomenon that affects overall plant performance. Biological nutrient removal is included in this discussion because the interaction and competition between microbial species affects both the sludge quality and determines the efficiency of removal or transformation of the organic and inorganic constituents in the wastewater.