Many STEM professional learning opportunities have been characterized as basic, disconnected, and ephemeral. This phenomenological mixed methods study addresses the growing demand for quality STEM educators by investigating the relationship between STEM professional development and teacher self-efficacy. While teacher efficacy has been identified as an important and essential element in the overall quality of STEM educators, this research provides a better understanding of the long-term influence of STEM professional development on STEM teacher self-efficacy. This study is the first comprehensive evaluation of a STEM professional development program within Southwest Florida and clarifies best practices related to increasing teacher self-efficacy in STEM for the purpose of growing the quality of STEM educators. Findings produced inform best practices for increasing teachers' sense of efficacy in STEM and the delivery of K-12 STEM professional development that is beneficial for the university, community, and school districts, all of whom have a vested interest in STEM education enhancement. [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.]