This chapter links some perspectives to the development of the brain and influences on it, and considers some findings on the psychobiology of conduct problems. Identification of early causes and indicators of altered neuronal function that might contribute to conduct problems requires an understanding of the development of normal structures. Sex differences in rates of aggression and disruptive behaviours within the normal range and at the extremes are very striking, and a biosocial model of conduct disorder will have to account for these. For instance, the long-term impact of maternal smoking in pregnancy on the risk of later anti-social behaviours appears to be stronger in males than in females. Considerable attention has been paid to the possibility that alterations in neurotransmitter levels underpin psychiatric conditions such as depression and attention deficit hyperactivity disorder (ADHD). The extent of environmental influences on brain development suggests potential both for vulnerability and plasticity.