The purpose of this longitudinal study was to explore the development of metacognitive knowledge in primary grade children (6–9 years, n=196) from pre-school to third grade. Knowledge about cognitive processes was obtained from children’s oral explanations. The results indicated that the average metacognitive knowledge of the primary grade children developed significantly during the first three school years. The children knew much about simple factors and strategies influencing cognitive activities, but after the first three school years, more complex strategies still remained unfamiliar to them. Moreover, the content analyses of children’s oral explanations revealed interesting differences in children’s metacognitive thinking. There appeared to be two groups of children with either non-developing or developing explanations. In addition, the children understood mental processing better in memory and learning activities, whereas comprehension processes were largely unknown to them.