This session will present the fundamentals of activated sludge and secondary clarifier operation with a focus on the development and use of State Point Analysis to define secondary clarifier capacity and RAS rates. Activated sludge concepts including F/M ratio, Solids Retention Time (SRT) and Aerobic SRT will be presented and illustrated using simulator demonstrations. Secondary clarifier operating criteria including Surface Overflow Rate, Solids Loading Rate and SVI will be presented as a precursor to discussing the background and use of State Point Analysis (SPA) techniques for the operation and troubleshooting of secondary clarifiers. Steady state and dynamic models based on both spreadsheets and simulators will be presented to illustrate secondary clarifier operating concepts and the SPA approach. Actual plant operating examples will also be included to demonstrate the use of the techniques presented.