The major purpose of this book has been to collect the knowledge that we now have of this field, including some indications of where it is going. Each of the authors has been encouraged to report the state of knowledge from an individual viewpoint. It is hoped that these chapters will be effective in conveying an impression of the state of the art in instructional technology. They may provide the substantive framework for the various techniques that make up the field of instructional technology, the problems yet to be solved by research and development, and an indication of the remarkable potentialities for the improvement of instruction.