Let me begin with what will surely strike many readers as obvious, but which needs to be said strenuously and incontrovertibly. Taken together, the contributions to this issue demonstrate conclusively that the adolescent's brain is different from both the child's brain and the adult's brain. It is different with respect to both morphology and function, and at the levels of brain structures, regions, circuits, and systems. It is different with respect to grey matter (Gotgay & Thompson, this issue), white matter (Paus, this issue), structural connectivity (Schmithorst & Yuan, this issue; White et al., this issue), and neurotransmission (Doremus-Fitzwater et al., this issue; Wahlstrom et al., this issue). It is different in ways that are revealed in studies of sleep (Feinberg & Campbell, this issue), electrophysiology (Segalowitz et al., this issue), functional imaging (Luna et al., this issue; Somerville et al., this issue), pharmacological challenge (Wahlstrom et al., this issue), and stress reactivity (McCormick et al., this issue). It is different in ways that are consistent with studies of juvenile rodents (Doremus-Fitzwater et al., this issue) and non-human primates (Dahl
