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Proportional-integral-derivative (PID) controllers are widely used in process control, and consequently they are described in most of the textbooks on automatic control. However, rather than presenting the overall design process, the examples given in such textbooks are intended to illuminate specific focused aspects of selection, tuning and implementation of the controller. This paper describes in detail the design of a PID controller for temperature control of a polymerase chain reaction (PCR) microreactor showing how different aspects, which necessarily are taught separately, interact in a real-world design. After setting the design targets by taking the hardware limitations into consideration, a continuous time controller, having two degrees of freedom, is designed by placing its dominant pair of poles using the root locus technique. Then, the integrator wind-up is addressed, the controller is translated into a discrete time version and, after implementation, the experimental performances are measured.