This exploratory study, in which the participants portrayed a critical role, aimed to discover the perceptions of professional development in changing educator practice at the secondary level with middle school teachers in an urban school district. The data collection process included semi-structured interviews with nine secondary teachers via Zoom and in-person interviews in the content areas of science and social studies and data from the Defined Learning platform, emphasizing practical insights for adult learners, policymakers at the district level, and future researchers. Social studies and science educators were interviewed about their perceptions of professional development facilitated by Defined Learning experts and its implications for teachers implementing project-based learning in their learning environments. Emerging themes and essential elements resulted from participants' interviews and data from Defined Learning. The themes identified were: Professional Development is Essential for Adult Learners, Alignment of Professional Development and Project-based Learning, and How Reflective Practice Influences Adult Learning. Findings suggested that adult learners supported implementing project-based learning through Defined Learning when it was purposeful and aligned with their pedagogy. This research also included evidence that future researchers can utilize in future studies addressing professional development and project-based learning. [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.]