Theories of language acquisition have highlighted the importance of adult speakers as active participants in children's language learning. However, in many communities children are reported to be rarely directly engaged by their caregivers. This raises the possibility that children in these communities learn language from observing 3 rd party interaction. The goal of this dissertation is to explore this possibility by observing and quantifying naturally occurring language input in one such community (Yucatec Mayan) and comparing this input to that heard by children in the United States. In the first section of the dissertation I provide quantitative evidence that one and two-year-old Mayan children do indeed receive less directed input than US children. In the second section of the dissertation I show that for Mayan (but not for US) children there are great increases in the proportion of directed input that children receive from 13 to 35 months. In the last section I explore the relationship between word types heard in Mayan children's input at 24 months and children's expressive and receptive vocabulary knowledge at 35 months. I demonstrate that word types directed to children, but not overheard types or all types, best predict children's later vocabulary. This suggests that talk directed to children is important for early word learning, even in communities where the bulk of children's early language input comes from overheard speech. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]