Project-based (PBL) learning has been found to improve student achievement, engagement, and interpersonal and intrapersonal skills; however, it is not widely implemented in K-12 schools in the United States. Prior research has established several barriers and challenges to implementing PBL in K-12 schools including challenges related to teacher beliefs, teaching practices, and school contexts. This multiple case study focuses on teacher beliefs and school contexts by seeking to understand what role the sources of self- and collective efficacy play in teachers' adoption of PBL into their classroom and how the sources of self- and collective efficacy and their role in teachers' adoption of PBL differ across three different secondary schools. Results of this study indicate that clear definitions of PBL and supportive structures such as school-wide professional learning, professional learning communities, and peer observations are needed for teachers to experience self- and collective efficacy for PBL implementation. Furthermore, the role of positive affective states such as joy and trust may play a more significant role in the development of self- and collective efficacy than previously thought and that more opportunities for vicarious experiences through peer modeling may be especially important for PBL efficacy. Implications for further research include additional research that focuses on the role of these enabling conditions for school-wide PBL as well as more research that clarifies how the sources of collective efficacy are operationalized in K-12 schools implementing PBL. [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.]