Some of the most intriguing chapters include the finding, by Baillargeon and colleagues, that physical aggression, when carefully measured on the basis of concretely observable forms—for example, fighting, kicking, and biting—evidences gender differences as early as ages 2 and 3 years. The chapter by Xie and colleagues offers a thorough discussion of measures and definitions of social aggression, which is consistently more common among girls than boys. However, although these and other authors submit that it is also important to reduce social aggression, they fail to demonstrate its impact on either the perpetrator or the victim and, in fact, provide data suggesting that socially aggressive girls are educationally more successful. One of the most easily read and intriguing chapters is by Artz, who describes themes that have emerged from her qualitative study of violent adolescent girls in Vancouver, British Columbia. She concludes that the most violent girls are caught in a trap of seeking to ally with those they perceive as being in power—namely, males. These girls resort to violence to protect their reputations and their romantic or sexual relationships and define their own worth primarily in terms of their ability to attract male peers. Finally, the chapter by Stack and associates on girls' aggression across the life course provides compelling evidence about the link of girlhood aggression, through complex pathways, to poor as well as adequate parenting. Of particular interest is the finding that although girlhood aggression was found to predict numerous subsequent difficulties, the group at the highest risk was girls who were highly aggressive and socially withdrawn.