This study explored the main effect association between a large scale federally funded preschool intervention and the social and emotional development of participants. Differential effects of program participation for child, family, and neighborhood level moderators were also considered. This is the first study of its kinds to find a significant association between preschool participation and child social and emotional outcomes in elementary and middle school. Data were drawn from the Chicago Longitudinal Study (CLS: Reynolds, 1991, 1998, 2000). The original sample of 1,539 in the CLS included the entire cohort of 989 children who attended the 20 Child-Parent Centers in preschool and kindergarten in 1985-1986 and 550 children of the same age who participated in an alternative all-day kindergarten program in 5 different Chicago public schools in similar neighborhoods. These schools were randomly selected from 27 sites participating in the Chicago Effective Schools Project (an intervention that offered all-day kindergarten among other services). As a consequence of living in school neighborhoods eligible for Title I funding, all children in this cohort were eligible for and participated in government-funded early childhood programs. The groups were well matched according to their eligibility for intervention, family socioeconomic status, gender, and race and ethnicity (see Reynolds, 2000). The study sample includes 1,378 primarily African American youth who participated in the CLS and had scores for 2 or more identifiable social and emotional competency indicators by age 15 (Niles et. al, 2003). Findings suggested that early intervention was associated with only a few shorter and longer-term social and emotional development. Only social adjustment in school at ages 7 and 8-9 were significantly different between groups. The strongest effect was seen for social adjustment in school at age 7 and 8-9 with d's of .45 and .33, respectively. The effect sizes for the longer-term , were relatively small, but some remained practically significant. These include social adjustment in school (d=.34), assertive social skills (d=.21), task orientation (d=.21), frustration tolerance (d=.22), and peer social skills (d=.24). The most frequently detected differential effects were for neighborhood poverty levels, sex of child, and children that attended CPC centers that offered a blended curriculum approach. Effect sizes ranged from .42 (boys benefiting more than girls on social adjustment age 7) to .14 (assertive social skills), most were at or above the level of practical significance.