The basic requirement for developing electrical schematics is to understand the process. The logic diagram is of particular use in describing a process when the activating input is related to an on-off signal. This chapter focuses on how programmable controllers and instrumentation are applied. The programmable logic controller developed out of the needs of the automotive industry. The industry required a control unit which was easily changed in the plant, easily maintained/repaired, highly reliable, small, capable of outputting data to a central data collection system, and competitive in cost to relay and solid-state panels. Process controllers are beginning to be used to make not only logic decisions but also control loop decisions. The chapter shows a process controller configuration that is often called direct digital control, since the control action is determined by the program within the controller. It explores the different types of control instrumentation: level, pressure, flow and temperature. The chapter summarizes the different instrumentation wiring methods.