Academic vocabulary knowledge is central to reading achievement, particularly for students who have limited experiences with academic discourse outside of school. Yet many teachers devote little time to focused instruction in this area, because they may lack critical meta-linguistic and pedagogical knowledge. The purpose of this longitudinal instrumental qualitative case study was to understand the experiences of three elementary teachers who engaged in extensive site-based professional development that included the study of Greek and Latin morphology in addition to effective strategies for instruction of academic vocabulary. Specifically, the study explored factors that influenced the teachers' efforts as they implemented and sustained changes to their practice over a three-year period. Data sources included in-depth and semi-structured interviews, direct observations of classroom practice, and analysis of instructional artifacts. Data analysis revealed that as a result of extensive professional development, all three teachers made significant changes to their vocabulary instruction. Their ability to sustain those changes, however, depended largely on situational dynamics in their school and classroom contexts. Over the three years participants were both supported and hindered by the interplay of a variety of job-related factors, including administrative expectations, collegial relationships, student populations, and curricular mandates. Study findings underscore the interdependent relationship between instructional practice and classroom, school, and district contexts. Implications for the educational community are included with suggestions for future research. [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.]