This paper uses data from the Head Start Impact Study (HSIS), a nationally representative multi-site randomized trial, to quantify variation in effects of Head Start during 2002-2003 on children's cognitive and socio-emotional outcomes relative to the effects of other local alternatives, including parent care. We find that (1) treatment and control group differences in child care and educational settings varied substantially across Head Start centers (program sites); (2) Head Start exhibited a compensatory pattern of program effects that reduced disparities in cognitive outcomes among program-eligible children; (3) Head Start produced a striking pattern of sub-group effects that indicates it substantially compensated dual language learners and Spanish-speaking children with low pretest scores (two highly overlapping groups) for their limited prior exposure to English; and (4) Head Start centers ranged from much more effective to much less effective than their local alternatives, including parent care. (author abstract)