Early childhood is an incredibly important time for the development of many key cognitive and social developmental tasks. Even in early childhood, engaging in aggressive behavior is associated with poor social and emotional outcomes. Hostility and aggressive thoughts, in particular, play a critical role in the development and maintenance of aggression. Understanding pathways to aggression and hostility (e.g., parents and family influences) and how they develop may be informative for determining how to intervene. These issues will be presented using findings from both past and ongoing research on social development during early childhood. Novel methods for assessing aggression and hostility in this period will be discussed. The implications of these findings for preventative interventions with children and parents will also be highlighted.