The premise of this book is that many of the experiences we encounter are novel, infrequent in our experience, or variable with respect to action to be taken. Others require decisions to be made in the face of ambiguous or incomplete information. Time pressure is frequently tight and penalties for failure are severe. Examples of such situations include investing in markets, controlling industrial accidents, and detecting fraud. These are environments that defy a definition of optimal performance. The authors refer to domains without criteria for optimal performance as 'competency-based' and we describe the behaviour of individuals who work in them by the term 'competence'. The chapters explore the proposition that metacognitive processes - thinking about the kind of thinking that a task requires - give structure to otherwise ill-structured tasks and is a fundamental enabler of competence at decision making. Such metacognition facilitates performance.