Various iterations of STEM have been in the forefront of national educational reform of science and math teaching over the last two decades. Most recently, STEM is being viewed as the integration of the sciences, math, technology, and engineering to solve authentic problems, rather than an individual collection of related subjects. This approach to educating students about core STEM subjects requires teachers who have expertise in content, pedagogy, and the integration of STEM content. This chapter discusses the complexity of issues that emerged during STEM programs designed for teachers in a university and two K-12 middle schools. Common themes are analyzed for implications to teacher preparation and professional development programs. The article concludes with the description of an approach to STEM teacher preparation that calls for (1) a reorganization of the knowledge base from the STEM disciplines and (2) professional learning programs that emphasize collaborative processes and links between content.