The widespread need to address both science, technology, engineering, and math (STEM) education and STEM workforce development is persistent. Underscored by the Next Generation Science Standards, demand is high for P-12 engineering-centered curricula. TeachEngineering is a free, standards-aligned NSF-funded digital library of more than 1,500 hands-on, design-rich K-12 engineering lessons and activities. Beyond anonymous site-user counts, the impact of the TeachEngineering collection and outreach initiatives on the education of children and their teachers was previously unknown. Thus, the project team wrestled with the question of how to meaningfully ascertain classroom impacts of the digital engineering education library and-more broadly-how to ascertain the impacts of teacher-focused P-12 engineering education initiatives. In this paper, the authors approach the classroom impact question through probing self-reported differentials in: (1) teachers' confidence in teaching engineering concepts, and (2) changes in their teaching practices as a result of exposure to (and experiences with) K-12 engineering education resources and outreach opportunities. In 2016, four quantitative and qualitative surveys were implemented to probe the impact of the TeachEngineering digital library and outreach on four populations of K-12 teachers' confidence and practices, including the frequency with which they integrate engineering into their precollege classrooms. Survey results document the teacher experience and perception of using hands-on K-12 engineering curricular materials in the classroom and help create a data-driven understanding of where to best invest future resources. The results suggest that the TeachEngineering curricular resources and outreach initiatives help teachers build confidence in their use of engineering curriculum and pedagogy in K-12 classrooms, impact their teaching practices, and increase their likelihood of teaching engineering in the classroom in the future.