More attention and resources have been devoted in recent years to early childhood development (ECD) in low- and middle-income countries. Rigorous studies on the effectiveness of ECD-related programs for improving children's development in various dimensions in the developing world are scant. The authors evaluate an important ECD initiative of the Philippine government using longitudinal data collected over three years on a cohort of 6,693 children age 0-4 years at baseline in two "treatment" regions and a "control" region that did not receive the intervention. The initiative includes a wide range of health, nutrition, early education, and social services programs. The authors estimate its impact by using "intent-to-treat" difference-in-difference propensity score matching estimators to control for a variety of observed characteristics measured at the municipality, barangay, household, and child level and unobserved fixed characteristics, with differential impacts by age of children and duration of exposure to the program. There has been a significant improvement in the cognitive, social, motor, and language development, and in short-term nutritional status of children who reside in ECD program areas compared to those in non-program areas, particularly for those under age four at the end of the evaluation period. The proportions of children below age four with worms and diarrhea also have been lowered significantly in program compared to non-program areas, but there are effects in the opposite direction for older children so the overall impact on these two indicators is mixed. (Contains 15 tables, 1 figure, and 18 footnotes. Appended are: (1) Components of ECD Program in the Philippines; (2) Additional Tables and Analyses on Attrition; and (3) Additional Estimates.) [This paper was financially supported in part by: (1) National Institute of Child Health and Human Development (NICHD), the Office of Behavioral and Social Science Research; (2) the Global Development Network; (3) the Department of Social Welfare and Development (DSWD) of the Government of the Philippines; (4) the World Bank Research Support Budget ; and (4) a NICHD postdoctoral training fellowship for Ghuman through the University of Michigan. An earlier version of this paper was presented at the Population Association of American (PAA) Annual Meetings in Philadelphia on April 1, 2005 and at the World Bank.]