Some of the process variables that are commonly monitored on a thickener are torque, rake height, bed level, bed pressure, feed rate and density, underflow rate and density, settling rate, and overflow turbidity. Many of these are easily measured, while some can be difficult. Combining these signals into a coherent control strategy requires forethought and an understanding of the fundamentals of thickener operation. A wide variety of control strategies have been implemented on thickeners, using various combinations of sensors. In recent years improved flocculants, higher throughput rates per unit area, and desired higher density underflow concentrations have required the development of better control strategies to successfully operate sedimentation equipment. This has been complicated by plant expansions that have placed increased loads on existing sedimentation equipment. Successful control strategies consider the process goals, plant fluctuations, sensor reliability, and system response times. A historical review will be discussed followed by discussion of the latest developments in sensors, control equipment, and control strategies.