Wavefront sensors, described in Chapter 5, measure the condition of the wavefront and produce signals that represent the wavefront. Both zonal and modal sensors may be used in a particular system. It is up to the reconstructor to sort out the meaning of those signals, and then it is the control system that must determine how to treat the signals and relay them to the appropriate correction device. In Chapter 4, many system concepts showed the control system as a “black box,” that is, the controller received the signals from the wavefront sensor and magically translated them to signals for the correction devices. This chapter aims to remove the mystery of the controller and to describe the fundamental principles behind the adaptive optics control system.