T aim of this thesis was to explore how genetic and environmental influences affect brain maturation and cognition around puberty in an epidemiological sample of children from the general population in the Netherlands. For this purpose, a large cohort of children was studied within a three year interval. The sample was recruited from the Netherlands Twin Register (NTR) and consisted of 112 families at baseline, including 9-year-old twins and an older sibling. There were 89 families (80%) who returned for the follow-up assessment. Data on cognitive abilities were collected with psychometric IQ tests and a comprehensive neuropsychological testing protocol. Brain anatomy was acquired by 1.5T magnetic resonance imaging (MRI).