Background: Behavior and socioemotional development are crucial aspects of child development. Methods: A total of 2505 children born at <27 weeks' gestation was evaluated at 18 to 22 months' corrected age between January 1, 2008 and December 12, 2012 (86% follow-up). The Brief Infant and Toddler Social and Emotional Assessment was used to evaluate behavioral and socioemotional problems. Cognition and language were evaluated by using the Bayley Scales of Infant and Toddler Development, Third Edition (Bayley-III). Logistic regression analysis was used to evaluate for perinatal and demographic factors associated with behavioral problems (≥75th percentile) and delayed socioemotional competence (≤15th percentile). Structural equation modeling with bootstrapping was used to identify possible associated risk factors and Bayley-III scores as mediators. Results: Thirty-five percent (873) of children had behavioral problems, and 26% (637) displayed deficits in socioemotional competence. Male sex, public insurance, mothers with less than a high school education, and lower maternal age were associated with behavioral problems. Deficits in competence were associated with lower birth weight, public insurance, mothers with less than a high school education, and abnormal neuromotor exam. Bayley-III language and cognitive scores were significant mediators of the relationships between risk factors and both behavioral and competence scores (P < .05). Conclusions: Extremely premature children are at risk for behavioral problems and deficits in socioemotional competence. Sociodemographic factors were associated with both socioemotional competence and behavioral problems. Deficits in socioemotional competence were also associated with neuromotor abnormalities and cognitive and language function.