This study examined the effect of Vygotskian-based professional development on the CLASS Instructional Support Teacher Self-Assessment scores of preschool teachers in a rural setting. Specifically, it examined the efficacy of preschool teachers in their ability to provide instructional support to their students prior to participating in Vygotskian-based professional development and their teacher efficacy after participating in Vygotskian-based professional development. It also examined these teachers' developmental levels on the CBAM Stages of Concern Questionnaire before and after participation in Vygotskian-based professional development. In addition, this study examined these teachers' baseline CLASS Instructional Support scores prior to participation in the Vygotskian-based professional development. The significance of studying the effect of professional development that is grounded in the Vygotskian framework is that teacher practices that are rich in social interactions, focus on the child's zone of proximal development, use mediation tools to scaffold children's learning, as well as use repetition through multiple modalities have been associated with school readiness and positive lasting child outcomes. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]