There are rich theories and designs for general control systems, but usually, they will not lead to PID controllers. Noting that the PID controller has been the most popular one in industry for over fifty years, we will confine our discussion here to PID control only. PID control has been an important research topic since the 1950s and has caused remarkable activities for the last two decades. Most of the existing works have focused on single variable PID control, and its theory and design are well established, understood, and practically applied. However, most industrial processes are of multivariable nature. It is not rare for the overall multivariable PID control system to fail, although each PID loop may work well. Thus, the demand for addressing multivariable interactions is high for successful application of PID control in multivariable processes, and it is evident from major leading control companies who all ranked the coupling.