The efficacy of metacognition as theory-based instructional principle or technique in general, and particularly in mathematics, is explored. Starting with an overview of different definitions, conceptualizations, assessment and training models originating from cognitive information processing theory, the role of metacognition in teaching and learning is critically discussed. An illustrative training program in kindergarten demonstrates that explicit and embedded metacognitive training can have an effect on mathematics learning even in very young children. Theoretical and methodological issues for future research, and recommendations for mathematics educators, are analyzed within the framework of the Opportunity-Propensity and Universal instructional design framework, demonstrating the relevance of metacognition in the domain of mathematics teaching, impacting children's learning of mathematics and their active involvement in their learning process.