This chapter provides an introduction to real-time controllers. This can be helpful in understanding the capabilities of regulatory control and also in troubleshooting problems that might be encountered. The simplest controllers are on-off controllers. The output from this type of controller is either 0% or 100%. The most common controller type, the PID controller, consists of three elements: a proportional term, an integral term, and a derivative term. Each term is applied to the error between the measurement variable and the setpoint. The proportional term provides responsiveness to the error, while the integral term provides a dampening effect. The derivative term is reactive to the rate of change in the error. Some controllers leave out the derivative or proportional terms. PID controllers are based on the assumption that the relationship between the measurement variable and the independent variable in the control loop is linear. Adjustments for nonlinearities that are too great include: reducing the operating range to a linear region, modifying the PID algorithm, making nonlinear transformations to the input or output variables, or employing tuning parameter scheduling. Adaptive (heuristic) controllers are also described. These are often used when the signal-to-noise ratio of the measurement variable is large.