A longitudinal study was conducted to examine variations in caregiver input and infant attention in association with children's later lexical and syntactic skills. Fifteen infant-caregiver dyads were videotaped during naturalistic interactions when infants were 9 and 12 months old. Videotapes were coded for caregiver style and modality, and infant engagement. Caregiver communication that followed the child's focus was defined as "child-centered". Child-centered acts were further assigned to two categories based on their perceptual salience: (a) "caregiver contingent comments" (CCC, Rollins, 2003) were utterances referring to an object or activity in the immediate environment; b) "other child-centered" were utterances about perceptually distant referents, such as feelings or social routines. A third style category, "directive" utterances, was coded when caregivers attempted to direct the child's attention. The total number of words (TNW) caregivers used during all interactions with 9-month-olds was associated with language outcomes; however, within the three style categories, it was only the TNW in CCC that predicted children's vocabulary at 30 months. Infant attention was coded when the child and caregiver attended to the same object or event. Episodes were coded as "coordinated joint engagement" when the child alternated attention between the caregiver and their mutual focus. In contrast, engagements were coded as "supported joint" when the child did not overtly attend to the caregiver. Caregiver conversational style and modality were coded during all joint engagements. Utterances were coded as "multimodal" when caregivers used objects and "unimodal" when caregivers did not use objects. The TNW in multimodal CCC during supported joint engagement at 9 months was associated with children's lexical diversity at 30 months. However, caregiver use of other style and modality types (i.e. multimodal or unimodal other child-centered and directive) during joint engagements at 9 months was not associated with children's language outcomes. During coordinated joint engagement at 12 months, the TNW caregivers used within all style and modality categories was associated with a 30 month language measure. Results suggest that early in the language learning process, perceptually salient input assists lexical acquisition during supported joint engagement. Later, during coordinated joint engagement, a wider range of caregiver input supports word learning. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]