The present study examined sex differences in levels of physical, verbal, and indirect aggression amongst primary school children and their relationship with instrumental and expressive beliefs about aggression. Levels of aggression were examined using self ratings, peer ratings, and observations. The latter were collected during the mid-morning and lunchtime breaks using a wireless microphone and hidden video camera. Beliefs about aggression were assessed using modified versions of Campbell et als.'  EXPAGG questionnaire. The results revealed significantly higher levels of observed physical aggression amongst boys as compared to girls. However, although the means were generally in the directions predicted, there were no other significant sex differences, nor interactions between sex and age. The results also showed limited support for claims that instrumental versus expressive beliefs about aggression influence behaviour. Beliefs generally showed significant correlations with reported and observed levels of aggression, and three of these remained significant even after the variance associated with sex and age had been partialled out. On the basis of these results, we call for more longitudinal research while simultaneously acknowledging the possibility that children's beliefs about aggression and their aggressive behaviours may be shaped independently from one another. Aggr. Behav. 30: 123-145, 2004. © 2004 Wiley-Liss, Inc.