The Parents as Teachers (PAT) program is a parent-education program that includes home visiting and is designed to begin prenatally or at birth. Through home visits, visitors called parent educators help parents to strengthen their parenting skills and knowledge of child development and to prepare young children for school. This article describes the PAT program and reports the results of evaluations of two randomized trials of PAT: (1) the Northern California (Salinas Valley) Parents as Teachers Demonstration, which served primarily Latino parents in the Salinas Valley of California's Monterey County; and (2) the Teen Parents as Teachers Demonstration, which served adolescent parents in four counties in Southern California. The two evaluations revealed small and inconsistent positive effects on parent knowledge, attitudes, and behavior, and no gains in child development or health, when analyses compared the experimental and control groups overall. However, subgroup analyses in the Salinas Valley program indicated that children in primarily Spanish-speaking Latino families benefitted more than either non-Latino or English-speaking Latino families, with significant gains in cognitive, communication, social, and self-help development. Subgroup analyses in the Teen PAT Demonstration indicated that families that received both PAT services and comprehensive case management services designed to help mothers improve their life course benefitted most. Subgroup analyses in the Salinas Valley study suggested that children in families that received more intensive services benefitted more than children whose families received less intensive services. Results from that study suggested that home visits produced about a one-month developmental advantage per 10 visits for participating children.