The increasing diversity of young children enrolled in state pre-K and Head Start programs has prompted examination of varying impacts for identified subgroups of young children. We argue that questions of subgroup impacts and the processes that may account for them should be prioritized in future evaluations of these programs. Three subgroups at high risk of poor school performance provide the focus for our discussion: low-income children exposed to significant adversity, dual language learners, and children with special needs. We further draw upon new hypotheses regarding the kinds of processes most likely to support both short- and longer-term public preschool impacts as they apply to these subgroups. We conclude with a set of research recommendations aimed at identifying features of these programs that may render them especially effective in the context of today's increasingly diverse classrooms of young children. (author abstract)