To understand the development of relational aggression during early childhood, 91 girls and boys ("M" age=39.0; "SD"=7.6 months) and their teachers participated in an 18-month longitudinal study. Children were observed for relational and physical aggression during free play in four time periods. Individually administered interviews were conducted to provide peer reports of relational and physical aggression. Teachers completed measures of relational and physical aggression and peer rejection. Findings support the psychometric properties of the observational methods for use during early childhood. Results suggest that girls are more relationally aggressive than male peers and boys are more physically aggressive than female peers. Moreover, children primarily direct their aggressive behavior at same-sex peers. Finally, relational aggression was found to be moderately stable during early childhood and was associated with future peer rejection problems. Results are discussed in terms of the importance of developing methods to investigate behavior patterns for understanding the early development of and future social-psychological risks that may be associated with relational aggression.