Teaching is a complex activity that requires making ongoing multiple decisions and sporadic, responsive actions all while performing preplanned prescribed tasks. Evidence of certain aspects of teaching can be best assessed with a well-designed observation instrument. This instrument was designed to assess the enactment of the essential elements of implementing a Science, Technology, Engineering, and Mathematics (STEM) Project-Based Learning (PBL) activity in classrooms. The steps leading up to the instrument development and follow-up suggestions are offered to those who are engaging in STEM professional development including districts, schools, academies, service centers, and university partners. (Contains 1 figure.)