Vocabulary is important to reading achievement and overall school success; however, in spite of the research in support of vocabulary instruction, robust vocabulary instruction is lacking in many schools. The purpose of this study was to provide a team of three grade five teachers and an instructional coach with professional development on strategies for vocabulary instruction that was responsive to their immediate instructional goals. Further, we investigated how teachers interacted during the professional development to co-construct their understanding of effective vocabulary instruction over time. Two of the teachers and the instructional coach each had more than 10 years of teaching experience, while the third teacher had fewer than three years of experience. Using discourse and qualitative analysis techniques to examine transcripts of the professional development sessions, we found the teachers primarily discussed instructional strategies and co-constructed their understanding by asking and answering questions, reporting, and interthinking. We closely examined one teacher's shift in thinking regarding dictionary use in vocabulary instruction.