For some decades, theoretical and empirical research has focused on the phenomenon of metacognition and its overwhelming importance to human learning and performance. The real growth in theoretical and empirical studies about metacognition started with the work of Flavell at the end of the 1970s in the context of research on metamemory. The metacognitive concept has been very successful stimulating a lot of studies. The metacognitive research on reading peaked in the 1980s and has levelled since. Metacognition has more recently also been applied to mathematics. metacognition can be differentiated into two central components, namely metacognitive knowledge and metacognitive processes or skills. In the same vein, Brown (1978) distinguished metacognitive knowledge about the interaction between person, task, and strategies characteristics from the regulation of one's own cognitive activities. The purpose of this book is to help to summarise and clarify some of the issues on the conceptualisation, the assessment and the training of metacognition on mathematical issues in learners with and without mathematics learning disabilities. It presents a kaleidoscopic view on European research for the role of metacognition in mathematics performance.