Aberystwyth, WalesApproaches to quality management in software-based system engineering (as in ISO 9001) currently focus on the process rather than the product. The quality of the engineering is necessary (but not sufficient) to ensure the quality of the resulting system. To be effective, we must consider what is the nature of quality in a software-based system, how we can measure or demonstrate it, and what steps we can take to assure it. The paper seeks to answer these questions. Most studies of software-based systems have concentrated on high-quality systems, but the key points (and the levers of control) can be seen by looking also at low-quality systems. Not every system justifies the effort required to give high quality: in many cases, a prototype is good enough to use. Software contributes to the qualities a system has and to system quality, principally in a positive way by allowing operator mistakes and random hardware defects to be detected during operational use and treated automatically. The possibility of defects existing in the software itself drives the principal argument of the paper.Quality in a software-based system can be viewed as a negative property: the absence (or paucity) of defects in the operational system. A defect is any undesired characteristic of the operational system, even if that undesirability had not been anticipated.This concept recognizes that specifications as well as designs and implementations may be deficient, explains the difficulty of demonstrating quality, and suggests a way of measuring it (by the likely paucity of residual defects, arising from mistakes of production and finite coverage in checking). Steps to achieve quality follow from this analysis: first identify the desired qualities, and their visible negations as defects; then analyze the production technology to identify the kinds of mistake that may be made; and define checks to detect any faults that might have been produced. The quality thus depends on the range of defects identified and the intensity of checking that has shown zero defects after correction of those found.Confidence about the quality of the operational system rests on the analysis of the kinds of defect that may have been inserted during production, and escaped detection by checking. These are latent defects, which some (unexpected) combination of operational situations expose. The risk of a latent defect influencing the actual behavior arises from the likelihood of residual defects together with their severity in operational situations.