Two ways of making a decision are objectively using formal analysis and subjectively using intuition. Psychological research has shown that the latter leads to better decisions in complex situations. Intuitive decision-making uses simplified rules of thumb heuristics. In engineering terms, complexity implies complex systems. A set of rules and principles – heuristics – is presented as a guide for making subjective decisions in complex system contexts, one aspect of which is to provide quality control and guard against bias and error. An extended practical example of the use of these heuristics is given involving assessment of the safety of nuclear-powered ships.