The purpose of this study was to determine the impact of professional development designed to enhance vocabulary instruction on four first-grade teachers’ vocabulary instructional practices. The study incorporated a formative design methodology and the collection of baseline data on current instructional practices to inform the design of the professional development and subsequent intervention. According to Nagy and Scott (2004), word knowledge encompasses aspects and stages that surpass the common acceptance of what it means to know a word, definitional knowledge, and into the incorporation of that word into a child's working vocabulary knowledge. The teacher participants were provided with vocabulary instructional strategies through professional development with the goal of improving their vocabulary instructional practice. Following the professional development, the researcher initiated an eight-week intervention including observations, group meetings, and interviews to evaluate the impact of the professional development. Findings indicated that the professional development and intervention positively impacted the vocabulary instructional practices of the four first-grade teachers. Implications for future research include the benefit of using effective practices in professional development and a formative design framework to impact the instructional practice of primary teachers. / Ph. D.