The intent of this mixed methods study was to determine the impact of professional development on teacher efficacy beliefs. Variables such as years of experience and level of professional degree were also examined. For the purposes of this work, those who had experienced more than 50 hours of intense, focused professional development on a given subject were delineated from those who had not experienced such learning. In the quantitative portion of this two-phase sequential explanatory design, the researcher utilized the Teachers' Sense of Efficacy Scale to determine levels of efficacy amongst teachers in three of Missouri's largest school districts. Survey questions also pertained to years of experience, level of professional degree, and whether or not the participant had been a part of 50 or more hours of intense, focused professional development. Those who had this level of training could volunteer to participate in a qualitative interview to follow this first phase of the study. Analysis of the data proved that those who had the deeper, more sustained professional development had higher senses of efficacy when compared with their peers who had not had this experience. The statistical significance was small, however. Using the grounded theory approach, the qualitative interviews derived four themes that were important for professional development as it contributes to efficacy: an Attitude of Continual Improvement, the Need for Expert Modeling, the Importance of a Plan and Evidence of Student Success. The researcher ultimately recommended improvement of principal preparation programs to better highlight the importance of plans and results, as owned by teams of teachers. The literature, along with this study, are in support of the idea that this type of professional development contributes to collective efficacy. It was also recommended that teachers and school leaders become more aware of both individual and collective efficacy and its fragile nature. With public schools constantly examined with great scrutiny, teachers must understand this powerful construct that they control and its direct impact on 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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]