One of the goals of the Substrate Process & Technology group at IBM in Rochester, Minnesota is to provide disk programs with consistently high quality substrates. A substrate inspection system that uses laser light has been designed, built, and is being used at Rochester. This paper explains the optics and detection scheme of the inspection tool called the Substrate Defect Detector (SDD). The SDD has been useful in studies to improve disk quality and to solve substrate problems in both manufacturing and development environments. Some results of the studies are reported and explained. Several examples of how the inspection system has been used to help solve substrate manufacturing process problems are discussed. The design objectives, operation, and use of the SDD are explained.