This unit in the series prepared by the Australian Science Education Project is intended for students capable of abstract reasoning and as an introduction to other high level units in the series. The core activities suggest a number of activities that should lead students to recognize that a system is something that transforms an input into an output, emphasizing a consideration of what particular systems do, rather than how they do it. The basic concept is extended by examining different physical, biological and social systems in the options: "Systems that Control Themselves" (thermostats); "Trail a Snail" (responses to the inputs of light, touch, nature of substrate); "Robots and Computers" (human analogues indicate the need for precise input statements); "Pin-Head Pupils" (pupil responses to light entering the eye); "Feedback" (drawing with and without visual feedback); "Steady as She Goes" (physiological systems regulating temperature of mammals); and "Your Class System" (classroom viewed as a system). Instructions and suggestions for each activity, a diagnostic test after the core section, and an appended answers section are provided in a student book. The teacher's manual provides comments on teaching techniques, suggests appropriate references, and lists necessary equipment. (AL)