This chapter describes the thermodynamic and control characteristics of the process and the nature of the equipment and working fluids used in chillers. It focuses on minimizing the unit cost of chilling by optimizing the flows and temperatures of the heat-transfer fluids as they continuously float the coolant supply to match the load. The proper control and optimization of chilled water systems can only be accomplished if the plant piping layout allows for it. The most efficient chiller operation is obtained when the difference between the condensing and evaporation temperatures is the minimum and that will occur when the air flow through the condenser is unrestricted (no louvers) and the heat transfer area of the evaporator is fully utilized (evaporator flooded). If the chilled water flow rate is constant, the difference between chilled water supply and return temperatures is an indication of the load.