The purpose of the present study was to investigate the structure of metacognitive knowledge in fifth grade pupils and its relation to school achievement. Since the structure of metacognitive knowledge is controversially discussed with little empirical evidence for either its domain specificity or its domain transferability, the debate remains somewhat hypothetical up to date. In theory it is assumed that the development of metacognitive knowledge begins highly domain and situation-specific and becomes more flexible and domain-transcending with practice and experience (Borkowski et al. "2000"). As standardized measures to assess metacognitive knowledge of students in the age group under investigation were missing, newly developed instruments were applied to assess the domain-specific metacognitive knowledge in reading and mathematics as well as domain-transcending metacognitive knowledge. The influences of domain-specific and general metacognitive knowledge on school achievement in the two domains were analyzed. While findings on the structure of metacognitive knowledge indicate some degree of domain specificity, they also point out a strong relation between general metacognitive knowledge and domain-specific metacognitive knowledge. The cross-sectional relation between metacognitive knowledge and academic achievement were relatively low. Implications for future research will be discussed.