It is with great pleasure that I present as guest author this issue one of my long-time colleagues, John D. Lytle. Mr. Lytle is a well-known senior optical design engineer, deeply acquainted with optical fabrication techniques and problems. For about the last two years, he has geared himself particularly to state-of-the-art fabrication of plastic lenses, while being associated with rapidly growing M.U. Engineering & Manufacturing, Inc. In this article, he presents quite useful and interesting insights and solutions regarding optical tolerancing for glass and plastic optical systems, particularly emphasizing the differences involved between working with one or the other.