The purpose of bioprocess control is to ensure that the plant operates as designed. This chapter presents the fundamental principles for control of biochemical processes. Through examples, the selection of manipulated and controlled variables in the classical reactor configurations is discussed, so are control objectives and the challenges in obtaining good control of the bioreactor. The objective of this chapter is to discuss the bioreactor control problems and to highlight some general traits that distinguish operation of bioprocesses from operation of processes in the conventional chemical process industries. It also provides a number of typical control loops for different objectives. A brief introduction to the general principles of process control, the PID control algorithm is discussed, and the design and effect of tuning are shown in an example. Finally, a discussion of novel, model-free control approaches for bioreactors is provided.