After-school programs, such as a STEM Career Club, can promote student interest, engagement, and awareness of STEM majors and fields, as well as encourage teachers to become more knowledgeable and competent in STEM areas. In this dissertation study, two schools were selected from a larger NSF-funded project to participate in this study. Teacher- Coaches (T-Coaches) from two rural middle schools in the southeastern United States (U.S.) participated in teacher professional development (TPD) sessions and Professional Learning Community (PLC) meetings to prepare them to lead an after-school STEM Club. The Community of Practice (CoP) framework and Social Cognitive Theory are employed to investigate underlying factors that contribute to teacher interactions and preparations, and differing STEM program outcomes. Data from the Dimensions of Success (DoS) observation tool, the teacher belief interview (TBI), T-Coach participation and attendance at TPD, attendance and audio recordings from PLC meetings, and T-Coach card sorts were analyzed over approximately 6 meetings for 5 months. Findings are presented in two chapters. In Chapter Four, a comparative case study of the interactions of the teachers at two participating middle schools is analyzed. Results indicate that for each case, the club's T-Coaches interacted positively to prepare for club meetings and have a well-functioning CoP within their PLC. The first case (Northern Middle School) interacted in ways that aligned with the CoP framework (enterprise and repertoire), which led them to achieve, on average, desirable ratings on 7 of the 12 DoS dimensions. However, the other case (Southern Middle School), the T-Coaches interacted in ways that demonstrated more equal levels of enterprise, mutuality, and repertoire; this PLC had higher DoS ratings during the STEM Clubs in all dimensions (11/12 met desirable ratings). These findings suggest that high levels of all of the social learning characteristics within PLCs can support more exemplary STEM Club implementation. In Chapter Five, results from the two schools of teachers' beliefs and practices indicate that for STEM program success, the whole of the team is better than the sum of its parts. Since individuals' values on each team aligned with different DoS dimensions, it was more likely that each dimension would be represented during STEM Clubs. Findings suggest that it was necessary for two T-Coaches who valued a certain dimension to ensure a DoS dimension would be met on the DoS rating. However, it was not sufficient that T-Coaches only valued a certain dimension. The dimension was not met if the T-Coaches did not have the training and preparation to meaningfully act on their beliefs. Informed by factors from Bandura's Social Cognitive Theory, these T-Coaches carried out different behaviors at the STEM Clubs depending on their personal beliefs and values, and the environment. Five TPD participation scenarios, ranging from full to no TPD preparation, identified from the findings seemed to predict the quality of the STEM Club, based on DoS scores. The following conclusions can be drawn: 1) Professional learning community meetings aided in the development of T-Coaches' community of practice and preparation for STEM clubs; 2) A CoP with high levels of all of the social learning characteristics (enterprise, mutuality, and repertoire) led to more desirable club outcomes than a team with lower levels in any of these areas; 3) At least two people who have developed the content knowledge and relevant skills and who value club success were needed at club meetings to ensure STEM Club success; 4) Teacher-Coaches became more prepared to lead successful STEM Clubs through engaged attendance at TPD and PLC meetings; 5) Interdisciplinary teacher teams, including non-STEM teachers, can successfully lead STEM clubs if the individuals are able to learn the content/skills. [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.]