Despite millions of dollars spent annually on professional development (PD) for teachers, most training proves ineffective in changing teacher practice or improving student outcomes on a large scale. The ineffectiveness comes from a lack of content specificity, poor training design and delivery, misalignment of local needs compared to external mandates, and single-dose practices with little follow-through (Gulamhussein, 2013; Jacob & McGovern, 2015; Organization for Economic Co-operation and Development, 2019). More importantly, PD designers lack recognizing the needs and interests of teachers (Darling-Hammond et al., 2017; Guskey, 2021). These include ensuring that training starts with assessing the needs of its participants, creating a conducive environment, and providing voice and choice in selecting PD courses. Furthermore, conventionally delivered PD limits participants' ability to collaborate and reflect, lacks engagement with activities that are problem-based and immediately applicable in practice, and devalues evaluation of outcomes and future needs. This quasi-experimental study examined the effectiveness of including adult learning theory (ALT), specifically andragogy, in the design and delivery of PD. Andragogy is a theoretical framework of six assumptions or principles of adult needs and interests that impact learning (M. Knowles et al., 2020). It also includes a conceptual framework for applying these assumptions through eight instructional design and delivery elements. The study took place during a voluntary day of PD presented convention-style where participants could choose courses spread out over six 45-minute sessions. Unknown to the teachers, many of the classes were conventional (control), and others contained andragogical principles and design elements (treatment). After each course, participants could complete a Likert-style inventory to rate their satisfaction with each of the fourteen factors. Parametric tests proved statistically significant. In every case, participants rated their satisfaction with the ALT-enhanced courses higher than their conventional counterparts. In addition, the results parsed out four design elements that could predict participant satisfaction with PD effectiveness. Finally, the participants' ages and teaching experience impacted their satisfaction with andragogical PD. Implications from the results include advocating for and crafting PD to better meet teachers' needs and interests. Doing so may increase both teacher retention and student achievement. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]