The National Reading Panel (2000) identified vocabulary as one of the five skills with the most robust influence on reading ability. Early differences in vocabulary drive enduring individual differences in reading performance. High quality early childhood educational experience can have lasting positive effects on vocabulary development. This dissertation conducted a secondary data analysis of the Head Start Family and Child Experiences Survey (FACES) to understand the associations between classroom quality, process and structural quality, and vocabulary development in Head Start classrooms. The study included three indicators of process quality: instructional quality, frequency of instructional practices, and teacher's beliefs, and three indicators of structural quality: class size, teacher-child ratios, and teacher pre-service qualifications. There were three notable findings. First, teacher-child ratio was the only structural feature that shared a direct association with vocabulary outcomes. Second, the effects of teacher-child ratio and teacher education on receptive vocabulary were mediated by frequency of instructional practices. Finally, Concept Development mediated the associations between class size, teacher degree, and children's expressive, but not receptive, vocabulary performance. Future directions and implications are discussed. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]