The federal Early Head Start (EHS) program began in 1995, and a randomized trial was conducted to evaluate the efficacy of 17 EHS programs. In all, 3,001 low-income families (35% African American, 24% Hispanic, and 37% White) with a pregnant women or an infant under the age of 12 months were randomly assigned to a treatment or control group (with 91% of the treatment group receiving some services). Data were collected when the children were about 1, 2, and 3 years of age, and at age 5 (2 years after leaving EHS). Research questions examined (1) impacts of EHS at ages 2 and 3 (when services were being offered) and at age 5, and (2) contributions of early education experiences across children's first 5 years of life. Child outcomes included cognition, language, attention, behavior problems, and health; maternal outcomes included parenting, mental health, and employment. Overall impact analyses at ages 2 and 3 indicated that EHS benefited children and families: impacts were seen in all domains, with effect sizes of significant impacts ranging from 0.10 to 0.20. At age 5, EHS children had better attention and approaches toward learning as well as fewer behavior problems than the control group, although they did not differ on early school achievement. Subgroup analyses indicated that cognitive impacts were sustained 2 years after the program ended for African American children and language impacts for Hispanic children who spoke Spanish. Some significant family benefits were seen at age 5. Mediated analyses identified which child and family impacts at ages 2 and 3 contributed to the child impacts at age 5 (most relevant were earlier treatment effects on child cognition and on engagement with the parent). Growth curve analyses were also conducted. Although fewer than half the children enrolled in center-based preschool programs between ages 3 and 4, almost 90% participated in the year preceding kindergarten. A higher percentage of EHS than control children were enrolled. Nonexperimental analyses suggested that formal program participation enhanced children's readiness for school while also increasing parent-reported aggression. At age 5, those children and families who experienced EHS followed by formal programs fared best overall. However, the benefits of the two experiences were associated with outcomes in different ways. Benefits in language, behavior, and parenting were associated primarily with EHS; benefits in early school achievement were associated primarily with preschool attendance. (Contains 9 figures, 20 tables, and 18 notes.)