This chapter discusses the use of microcomputers to acquire signals from instruments, record the data, and take action based on the signals received. Microcomputers are becoming so inexpensive and ubiquitous that no water or wastewater treatment plant will be without this equipment. In a conventional water or wastewater treatment plant, many parameters of operation are measured and are usually displayed in a control room on panel meters or strip chart recorders. To control aspects of operation, say turning on or off a pump, signals must be sent from the microcomputer to the various devices. The BSR controller will send signals over the AC lines and turn on/off a small relay in a module attached to an AC wall outlet. On many control panels, pushbuttons are present which activate a circuit. These can certainly be used, and by putting a relay in parallel, the action can be carried out under computer control.