Many developmental theories that emphasize the importance of developmental task attainment imply that there is a degree of homogeneity in the developmental histories of children with serious disorders. From the perspective of developmental psychopathology, however, children identified as having serious emotional or behavioral disorders are likely to have followed diverse paths to those conditions. This study examined developmental task attainment histories of 554 children (ages 6 to 12) receiving mental health services and their association with aggressive behavior in middle childhood. The children, in spite of their common eligibility status for intensive services, had diverse experiences with 6 commonly cited developmental tasks. In addition, difficulties accomplishing specific tasks--poor bonding with a caregiver in infancy and poor behavioral regulation in early childhood--and patterns of difficulties with task attainment over time were associated with assaultive/threatening behavior in middle childhood. Children who did not encounter difficulty attaining any of the examined tasks or who had difficulty with tasks only in middle childhood had better functioning than children with task difficulties that began in infancy or early childhood. The diversity of developmental histories observed in the clinical population and their association with functioning suggest that research is warranted into how interventions may need to vary for children with similar presenting problems but different developmental histories.