The Home Instruction Program for Preschool Youngsters (HIPPY) is a two-year home-based early education intervention program designed to help parents with limited formal education prepare their four- and five-year-old children for school. This article begins with a brief overview of the HIPPY program and then presents the findings from a series of interconnected research studies, including a two-site, two-cohort evaluation in New York and Arkansas, a one-site case study, and a three-site qualitative study. With respect to program effectiveness, results varied across the New York and Arkansas sites and across participating cohorts at each site. For Cohort I, children who had been enrolled in HIPPY scored higher than children in the control/comparison groups on measures of cognitive skills (New York), classroom adaptation (New York and Arkansas), and standardized reading (New York); and more children were promoted to first grade (Arkansas). For Cohort II, comparison group children outperformed HIPPY children on school readiness and standardized achievement at posttest (Arkansas). Analyses to account for the differing results between cohorts were inconclusive. Qualitative analyses revealed considerable variation in parent involvement in HIPPY. Program staff identified four patterns of attrition from HIPPY: (1) early attrition within the first month after enrollment, (2) attrition between the program's first and second years, (3) attrition due to changes in the life circumstances of participating families, and (4) attrition due to turnover among the home visitors. Families were more likely to participate in in-home than out-of-home aspects of the program (for example, group meetings), but different family characteristics were associated with participation in the in- and out-of-home aspects of the program. The authors conclude with recommendations for future practice and research.