The findings indicated that romantic relationship security was associated with externalizing behavior problems in both adolescents and emerging adulthood, with a statistically stronger relation during emerging adulthood and during adolescence. Significant associations between romantic relationship involvement and externalizing behavior problems from age 19 to 23 were accounted for by romantic relationship security, indicating the primary importance of the quality of the relationship. A statistically significant but indirect relation could not be accounted for by early childhood parenting, middle childhood peers, adolescent family function, or overall competence during the transition from adolescence in to emerging adulthood. A provocative implication of these findings is that romantic relationship security may act as a protective factor for externalizing behavior problems in emerging adulthood. Becoming an all romantic relationship during emerging adulthood appears to be associated with lower levels of externalizing behavior problems. The present findings underscore the importance of considering a developmental perspective to the link between romantic relationships in externalizing behavior problems during adolescence and emerging adulthood. The present findings further provide evidence that one romantic relationship process, romantic relationship security, is more strongly related in behavioral outcomes during emerging adulthood and during adolescence to further investigate a developmental distinctiveness of adolescent and emerging adulthood romantic relationships, future studies should examine whether the results for externalizing behavior problems are indicative of the significance of romantic relationships for other behavioral outcomes, such as individual well-being and depression. The sample included 143 participants from the Minnesota Longitudinal Study on Parents and Children, an ongoing study of competence and psychopathology during the first three decades of life. Tables, references