This Head Start research project was part of a larger, longitudinal Early Head Start (EHS) study being conducted in the Juniper Garden Children's Project of the University of Kansas and the EHS Program, Project EAGLE, in Kansas City, Kansas. This quantitative research study examined the language development of Mexican American and Mexican immigrant children and explored factors that influence their language growth in their bilingual-bicultural environments. Project participants were selected from the longitudinal EHS sample, including 30 children, from 10 to 36 months of age, and their families. All participants had previously been randomly assigned to either an EHS group or a comparison group. The objectives of the study were to examine (1) the trajectories of children's English and Spanish vocabulary development in bilingual home environments; (2) the relationships among parents' level of acculturation, their expectations for their children's bilingual language development, and their observed language interactions with their children in English and Spanish; (3) the relationships among family factors, primary caregivers' amount and quality of interactions in English and Spanish, and children's language outcomes; and (4) the effect of EHS program participation on caregiver-child interactions in English and Spanish and children's outcomes in both languages. Child and family measures were administered every 4 to 6 months until children reached 36 months. The study examined children's language development, and the factors that influence this development, over 3 years.