The journey towards zero-defect manufacturing is a long and checkered one. Checkered because some quality defects are tenacious and they persist even after the most sophisticated quality management techniques have been deployed. Most available techniques work within the confines of known cause-effect relationships to identify the root cause of quality problems. But when a problem arises where the root cause lies outside the bounds of known cause-effect relationships, these techniques fail. This book describes the evolution of a technique, called Differential Diagnosis, which charts a dramatic, new path towards diagnosing root causes. It involves deep observation of the problem, emphasizing the absence of phenomena rather than the presence, thus permitting the elimination of a whole range of possible causes. In the process, the area of search is dramatically reduced and the root cause becomes easily identifiable. The book uses case studies that are narrated against a backdrop of a professor-student relationship, making it eminently readable as well as informative. The case studies are drawn from real life applications of the technique by the authors. They are very simply presented, with minimal technical data, making the material accessible to readers from all disciplines.