Research Findings: This study examined the stability of preschoolers' peer play behavior across the school year and the relations between emotion regulation, receptive vocabulary, and the trajectory of social competence deficits. Participants were 331 preschool children attending Head Start; they were primarily African American and from a low-SES background. Peer play behavior was moderately stable from fall to spring. Analyses revealed that emotional lability in the fall was associated with consistently maladaptive and declining social competence. Furthermore, children who exhibited stable maladaptive behavior had lower receptive language skills and emotion regulation in the fall than children who exhibited consistently adaptive behavior. Preschool children with comorbid externalizing and internalizing behaviors during peer play were at the greatest risk for consistent peer play difficulties or declining social competence over the course of the year compared to their peers. Practice: The present study informs practices for identifying at-risk preschoolers shortly after entry into an early education experience. Moreover, the findings suggest that without effective interventions, those at-risk children are likely to exhibit consistently poor social competence over time. Implications for the use of early intervention and prevention targeting specific behavioral and peer problems are discussed. (Contains 2 figures and 3 tables.)