This review begins by outlining the historical discussion about the relative importance of fostering cross-curricular competencies versus domain-specific prior knowledge as central goals of education. Metacognition and prior knowledge are then introduced as constructs representing these two goals; their development and effects on learning outcomes are described from a theoretical perspective. Empirical research on metacognition and expertise--and especially on mechanisms of acquisition--is then presented, illustrating commonalities between the two concepts and drawing attention to the need to take interactions between the two into account when analyzing their impact on learning outcomes. Most of the research discussed in the article draws on cross-sectional data. Systematic longitudinal research of combined effects in different subject domains is required to gain a better understanding of developmental mechanisms and stage-specific effects.