There have been recent calls for the integration of STEM (science, technology, engineering, and mathematics) in K-12 education. In this collective case study, we describe three teachers' implementations of a new STEM curriculum that integrates mathematics, engineering, and technology. These teachers ranged in their faithfulness to the curriculum authors' intentions. The common conceptions that influenced teachers' implementations were (1) negative beliefs about students and their abilities (e.g., "my students can't do that"), which caused teachers to stray from the authors' intended curriculum; (2) nontraditional beliefs about teaching, which caused teachers to use pedagogical techniques similar to those envisioned by the curriculum authors; and (3) a lack of subject matter knowledge, which caused one teacher to create a teacher-centered classroom that went against the authors' pedagogical intentions. Overall, this study illustrated how intricate and involved the implementation of a STEM curriculum can be.