This chapter begins with a description of the toilet technology, the valves for maintaining the water level in the toilet tank. The toilet tank is comprised of two primary, independent control mechanisms, the valve for controlling water flow into the tank and the valve for controlling water flow out of the tank, both relying on the same variable for feedback control. The primary concern in the development of the technology was the amount of water used by the toilet and its subsequent impact on the water supply and sewage collection and treatment systems. Ktesibious recognized that accurate time keeping requires precise control over water flow and maintenance of a constant water level in the container. Combining both sensing and control in the same device, the flush valve is a modem variation of Ktesibious cone-shaped float that controls the amount of water flowing out of the toilet tank.