An examination of the written evidence of the effects of preschool programs on disadvantaged children and their families is presented. Hundreds of studies were reviewed to determine what kind of justification they provide for continued support of federal, state, and other publicly financed preschool programs. The Head Start and ESEA programs have been reaching about 530,000 children of low-income families or neighborhoods yearly. Two major state-supported programs in California and New York together involve about 30,000 children beyond those in Head Start and ESEA Programs. Findings include: (1) Public preschool programs have been successful in changing intellectual and social behavior of disadvantaged children in positive directions over the short run; (2) Uncertainty about effects on children's social and emotional development stems not only from the paucity of reliable measurement but also from lack of concensus about what constitutes positive change; and (3) Participation of the parents in workshops and meetings at preschool centers has not been shown to make reliable changes in parents' attitudes about themselves and their own situations, but measures almost always indicate positive feelings toward the preschool program and positive changes in attitude toward school. It has been concluded that these preschool programs promote growth and development in disadvantaged children and that such programs might be justified as models for research and reform. (Author/CK)