This study explored 20 tenured professors' teaching improvement efforts in introductory undergraduate science, technology, engineering and mathematics (STEM) classrooms at two major American research universities (MRUS). It identified the mechanisms central to these professors' efforts to improve their undergraduate teaching and the influences and resources shaping those efforts. Despite the billions of federal dollars invested in STEM educational enhancement, STEM professors' teaching improvement efforts are little understood. This research examined this problem through analysis of 40 in-depth interviews (two per participating professor), 36 observations of professors' classroom teaching, and hundreds of documents representing professors' instructional efforts and career progression. The following propositions summarize key study findings: First, contrary to prevailing views, some STEM professors in MRUs do engage in efforts to improve their introductory teaching. Second, some of these professors employ creative, strategic, and systematic designs in so doing. Third, STEM professors' teaching improvement efforts are contextualized by internal and external forces that may facilitate or stymie their teaching improvement endeavors. Finally, STEM professors may be aware of institutional and external resources available to them as supports toward introductory STEM teaching improvement. Taken together, the data suggest that some university STEM professors do engage in efforts to improve their teaching and that such effort may be more common than popular opinion holds. The study revealed the inaccuracy of common beliefs and policy assumptions that the large majority of MRU-based STEM professors neglect their introductory teaching, do not care about it, lack knowledge about students and pedagogy, and prefer use of (and consistently rely on) conventional teaching approaches. To the contrary, all 20 participating professors were found to devote extensive energy toward improving their introductory teaching. Further, all 20 participants indicated extensive knowledge of introductory STEM subject matter, students, and pedagogies. The study also identified over 30 innovative pedagogies that participating STEM professors employed in their classrooms. Drawing on these findings, the study calls for public and policy-level reconsiderations of what it means to be a research-active STEM professor, including revision of extant ideas about how these professors invest their energies and time, and what they care about.