Because of the documented positive impact of high teacher efficacy on student success, it is critical that schools consider methods to increase teacher efficacy. The most common way to effect positive change in a school is through quality professional development. One type of effective professional development is action research–a method wherein practitioners investigate questions relevant to their own situations with the purpose of effecting a positive change. This research investigated the impact of participation in action research on teacher efficacy. The research design followed a mixed methods, multiple case study model. Data analysis showed that participation in action research did impact teacher efficacy, especially in instructional practices. A teacher’s willingness to continue as a teacher-researcher was also positively impacted for some participants.