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Traditional accident models were devised to explain losses caused by failures of physical devices in relatively simple systems. They are less useful for explaining accidents in software-intensive systems and for nontechnical aspects of safety such as organizational culture and human decision-making. This paper describes how systems theory can be used to form new accident models that better explain system accidents (accidents arising from the interactions among components rather than individual component failure), software-related accidents, and the role of human decision-making. Such models consider the social and technical aspects of systems as one integrated process and may be useful for other emergent system properties such as security. The loss of a Milstar satellite being launched by a Titan/Centaur launch vehicle is used as an illustration of the approach.