This randomized control trial evaluated the impact of a year-long professional development (PD) program on elementary teachers' CS content knowledge, self-efficacy, and CS implementation. It also investigated teachers' PD experiences and expectations. The findings suggest that treatment teachers' self-efficacy for teaching CS significantly improved as a result of the PD, and a statistically significant difference was found between treatment and control teachers' self-efficacy by the end of the PD. A higher percentage of treatment teachers than control teachers reported implementing CS-integrated lessons in their classrooms, and treatment teachers identified effective activities to engage students in learning CS and several barriers they experienced in CS integration. Treatment teachers' CS content knowledge remained the same from before to the end of the PD year, but a statistically significant decrease was observed for control teachers' CS content knowledge from pre- to end of year. Between group year-end differences were not statistically significant. At the end of the PD year, more treatment teachers demonstrated greater CS knowledge on 3 of 5 items than control teachers. Findings highlight the potential need for continual, sustained, and relevant CS PD over the academic year. Further study is underway with additional cohorts of the PD model.