The premise of this book is that most activity in everyday life and work is based on tasks that are novel, infrequent in our experience, or variable with respect to the action to be taken. Such tasks require decisions to be made and actions taken in the face of ambiguous or incomplete information. Time pressure is frequently great and penalties for failure are severe. Examples include investing in markets, controlling industrial accidents, and detecting fraud. The environments in which such tasks occur defy a definition of optimal performance, yet the benefits of successful decision making are considerable. The authors refer to domains without criteria for optimal performance as competency-based and describe the able behaviour of individuals who work in them by the term competence. The chapters examine the propositions that metacognitive processes give structure to otherwise ill-structured tasks and are fundamental enablers of decision-making performance.