The secondary clarifier plays a critical role in the success of the activated sludge wastewater treatment process by producing the effluent stream and by concentrating the mixed liquor. Secondary clarifier optimization requires consideration of both performance and capacity. Performance requires that the clarifier produce acceptable effluent quality – a function of biological flocculation, physical flocculation and hydraulics. Capacity requires that influent solids be transmitted to the floor at a rate greater than they are fed to the clarifier – a function of the clarifier surface area, the overflow and underflow rates, and the feed solids concentration and settling characteristics. Exceeding capacity limits results in storage of solids in the clarifier to the point of interfering with hydraulics or flocculation. This paper describes techniques to evaluate both secondary clarifier performance and capacity.