Engineering is Elementary (EiE) is a curriculum project of the Museum of Science, Boston, that promotes and supports engineering literacy and educational equity for all children. Building on the success of its award-winning curriculum for grades 1-5, the team has recently turned its attention to Wee Engineer, a research-based engineering curriculum for preschool programs. The cornerstone of the new curriculum is an age-appropriate engineering design process (EDP) that reflects both authentic engineering practices and the pedagogy of the preschool classroom. This article introduces the research and the design decisions behind the Wee Engineer EDP, and highlights how other developmentally appropriate elements in the curriculum support early STEM learning.