STEM, the integration of Science, Technology, Engineering, and Mathematics is increasingly being promoted in elementary education. However, elementary educators are largely untrained in the 21st century skills of computing (a subset of technology) and engineering. The purpose of this study was to better understand elementary teachers' self-efficacy for and beliefs about teaching computing and engineering. An entire faculty of a US-based elementary school participated in a year-long series of weekly professional development trainings in computing and engineering. Researchers collected quantitative data through a survey designed to assess teachers' self-efficacy and beliefs towards the integration of computing and engineering and compared responses with a demographically similar Title I school in the same city. Additional qualitative data was collected through semi-structured interviews and documented observations. Researchers found that between the two schools, self-efficacy and beliefs toward computing and engineering were likely influenced by professional development (p < 0.05). Through interviews, teachers attributed changes in self-efficacy and beliefs to the trainings. Although all teachers reported higher beliefs about the importance of computing and engineering, their self-efficacy for teaching these varied widely. A grounded theoretical analysis revealed this difference was likely attributed to each teacher's level of implementation, background, and willingness to experiment. We discuss how these factors may affect the professional development of elementary educators in preparing them to teach computing and engineering-related topics.