During a 2019 workshop, the American Association of Universities (AAU) convened experts involved in leading, researching, evaluating and funding efforts to transform undergraduate STEM (Science, Technology, Engineering, and Mathematics) teaching across all sectors of higher education. The goal was to synthesize essential issues and challenges in designing, leading, and researching institutional and multi-institutional transformation projects to improve undergraduate STEM education. The workshop was designed to identify critical themes necessary to understand when examining the outcomes of a reform effort, and when examining the change process of a reform effort. In addition, the workshop created a forum to provide recommendations for the continued advancement of the Improving Undergraduate STEM Education (IUSE) program through the National Science Foundation's (NSF's) Education and Human Resources, Division of Undergraduate Education. This workshop report serves as an executive summary of key findings and recommendations that emerged from an analysis of small-group activities and notes from whole-group discussions. The report contains four reflections (Ann E. Austin, Karl A. Smith, Jacqueline R. Roberts, and Michael J. Pullin) on the key themes and recommendations written by workshop participants situated in different sectors of higher education, including a higher education research scholar, a faculty member at an R1 university, a faculty member at a liberal arts college, and a faculty member at a community college.