In order to identify the active ingredients in an effective professional development intervention focused on enhancing preschool vocabulary instruction, this study examines the frequency with which teachers and children discussed theme-related vocabulary words during shared book reading. Head Start teachers received 1 year of training focused upon early vocabulary development. Children's vocabulary skills were assessed in the fall and spring of the school year. In spring, teachers read a storybook to their classroom, and teachers' and children's remarks about theme-related vocabulary during the reading -- including contextualized and decontextualized statements as well as verbatim repetitions of one another's statements -- were coded. Practice or Policy: Results of multilevel models showed that more frequent references to thematic vocabulary by teachers were linked to stronger child vocabulary development. Although children's vocabulary references were not uniquely predictive of vocabulary learning, teachers' repetition of children's remarks contributed to children's vocabulary gains. (author abstract)