Proportional-Integral-Derivative (PID) controllers are still the most common control algorithms used in the process industry. PID is a reduced complexity controller that provides the bare essentials for control: an integrator for low-frequency disturbance attenuation, and two-zeros-worth of phase lead for stabilization and phase margin adjustment. Practical experience shows that its tuning can be accomplished with very little information about the plant from the point of view of standard design techniques. This celebrated feature carefully underplays the fact that as a limited degree of freedom controller, the PID may be unsuccessful in controlling arbitrary plants and the tuning techniques may become progressively more complicated as the class of plants is expanded. For example, it is quite straightforward to show that PID controllers can always stabilize a single integrator and that they cannot stabilize a chain of three integrators. Furthermore, techniques that have a well established track record for the typical process control application, e.g., a heating process, are shown to fail when the plant contains flexible modes, e.g., a pendulum with flexible shaft. It is in these cases where PID control must ultimately escape the back-of-the-envelope calculations and the nearly-model-free framework. The most popular controller can now utilize the most recent computational methods and design understanding. The benefits are in the hardware simplicity, including anti-windups and fast execution times, and ease of scheduling.Many methods for PID tuning are found in the literature, and [1] is an excellent introduction. Typical examples include an analytic derivation of the tuning, based on