Quality can determine a software product?s success or failure in today's competitive market. Among the many characteristics of quality, some aspects deal directly with the functional correctness or the conformance to specifications, while others deal with usability, portability, and so on. Correctness is typically the most important aspect of quality. Even for market segments in which new features and usability take priority, such as software for personal use in the mass market, correctness is still a fundamental part of the users? expectations. This article classifies several quality-evaluation models.