In its most general sense, a system is a group of components integrated to accomplish a purpose. The heart of an educational system is the instructional system. An instructional system is an integrated set of media, equipment, methods, and personnel performing efficiently those functions required to accomplish one or more learning objectives. An objective is the behavior the student is expected to display after having learned. Several functions have been identified as being necessary for a complete instructional system. The critical functions are practice of performance, practice of knowledge, presentation of knowledge, student management, and quality control. These functions may be performed by a variety of components, including people, machines, books, and different kinds of equipment. Although the instructional system is clearly the heart of an educational system, other systems are necessary. These include record-keeping, feeding, supply, financial, transportation, and communication systems. A variety of simulation techniques have been developed to study systems and to improve system effectiveness. (Author/DN)