Young children are capable of engaging in STEM investigations when they are guided by skilled and knowledgeable teachers. However, many elementary teachers may lack sufficient STEM content knowledge and report feeling unprepared to teach STEM content. Two university faculty members in mathematics and science education, worked to cultivate and advance two designated Elementary STEM-Focused professional development schools through a two year series of an after-school STEM professional development (PD) Program. As the STEM PD Program progressed, it became evident that teachers were interested in and needed more experiences with the elements of the engineering process for young learners. With this in mind, several of the PD sessions were designed to highlight the engineering process and allow teachers to experience various activities that would engage young learners. To examine how this focus on the engineering process impacted the teachers in this STEM PD Program, a research study was organized during year two of the STEM PD Program. The results of this study provide evidence that this program had a positive influence on the teacher participants' engineering teacher efficacy and implementation of engineering lessons and activities within their classrooms.