Whole brain volume and head size, estimated by the measurement of occipito-frontal circumference (OFC), have been implicated in a variety of developmental disorders. The goals of this chapter are to review the (a) relation of head growth to general neurodevelopment, (b) growth patterns in premature infants and relation to outcome, (c) overgrowth and its relation to neurodevelopmental disorders, and (d) relation between psychological syndromes and growth patterns. Additionally, this chapter (e) discusses head circumference size and growth in pathologic conditions, focusing on atypical development in overgrowth syndromes and pervasive developmental disorders such as autism. Last, this chapter (f) reviews some practical and technical issues involved in the use of head circumference data for clinical documentation and for research. In summary, increased HC and growth have been found to be associated with higher IQ in neurotypical populations. However, this relationship may more accurately be represented by a U-shaped function, with developmental disorders associated with high and low extremes. In addition, a number of genes have been found to be associated with macrocephaly, which may also contribute to developmental disabilities, including autism. Thus, head circumference may be a useful biological marker for assessing developmental risk and for stratification in genetic analysis.