Most research on delinquency and criminality suggests that physical aggressiveness during childhood is a critical predictor of later antisocial and violent behaviors. This research encourages further study into the emergence and the development of aggressive behaviors. Most existing studies are based on samples of school-aged children or teenagers. Some research has highlighted the decline of aggressive displays during these periods. However, few studies have focused on the emergence of such behaviors. Consequently, little is known about the onset and development of physical aggression before starting school. Recently, Tremblay et al. (1999) have examined this topic. Using the same methodology, we interviewed 100 women about their 17-month-old child. Our results indicated that physical aggressiveness is a behavior trait that generally emerges prior to childhood and adolescence. By 17 months of age, the onset of physical aggression is reported in nearly 90% of the children. Moreover, the sex of the child and the presence of a sibling appeared to influence the age of onset.