National data shows that despite gains over the past twenty years, females remain underrepresented in the engineering workforce. Education's role in supporting the engineering pipeline cannot be ignored. Middle school teachers who lack integrated science, technology, engineering, and mathematics (iSTEM) efficacy are less likely to engage students, especially girls, with engineering lessons, thus decreasing STEM interest in middle grades. This study is built on the transformative power of professional learning communities (PLCs) presented in the literature. Three Grade 7 life science teachers at a public middle school in Massachusetts participated in an eight-week iSTEM and engineering education (EE) PLC to increase iSTEM teacher efficacy. During the iSTEM/EE PLC meetings, educators participated in engineering education and gender-inclusive pedagogy professional development. Teachers put their learning into practice by developing and implementing an iSTEM lesson with an engineering focus. This mixed methods study showed that the iSTEM/EE PLC intervention increased average teacher iSTEM efficacy by 12.9%, with improvements seen for both the iSTEM and EE constructs. In addition, improved PLC structure and teacher collaboration supported changes in iSTEM efficacy. Study findings indicate that iSTEM/EE PLCs can improve teacher efficacy and should be more widely implemented. This, in conjunction with vertically aligned engineering learning opportunities, mentoring, and engineering enrichment programs, will provide girls with high-quality experiences that help them see engineering as a viable career option. [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.]