Science, Technology, Engineering, and Mathematics (STEM) education is important on a national, regional, local, and individual level. However, there are many diverse problems facing STEM education in the US, one of the most critical is the limitation of current measurement tools and evaluation methodologies. The development of a common measurement system is an important step in addressing these problems. This paper describes the conceptualization stage of the development of a common measurement system. The resulting STEM Common Measurement System includes constructs that span from student learning to teacher practice to professional development to school-level variables. The authors detail the constructs and measurement tools associated with each construct. The interconnections within the STEM Common Measurement System are also discussed.