Many of the papers in this collection reaffirm some established principles about adolescent brain development, but with important added caveats and nuances. For example, evidence continues to accumulate that brain maturation continues throughout adolescence, with some of the most significant changes taking place in prefrontal areas. It is now well established that there is a decline in grey matter and an increase in white matter in these regions during adolescence, but as several authors note, there is some evidence that these changes are not due solely to synaptic pruning and myelination, as had been widely assumed. With regard to the first, it now appears that some of the change in the grey matter density that had been attributed to synaptic pruning may instead be due to other cellular processes as well as increases in white matter, which would affect estimates of grey matter density (Gotgay & Thompson, this issue). Moreover, as Paus (this issue) points out, the extent to which increases in white matter are due to changes in myelin versus changes in axonal diameter is not yet known; although
