STEM education reform in higher education is framed around the need to improve student learning outcomes, increase student retention, and increase the number of underrepresented minorities and female students in STEM fields, all of which would ultimately contribute to America's competitiveness and prosperity. To achieve these goals, education reformers call for an increase in the adoption of research-based "promising practices" in classrooms. Despite efforts to increase the adoption of more promising practices in classrooms, postsecondary instructors are still likely to lecture and use traditional teaching approaches. To shed light on this adoption dilemma, a mix-methods study was conducted. First, instructional practices in large introductory STEM courses were identified, followed by an analysis of factors that inhibit or contribute to the use of promising practices. Data were obtained from classroom observations (N = 259) of large gateway courses across STEM departments and from instructor interviews (N = 67). Results show that instructors are already aware of promising practices and that change strategies could move from focusing on the development and dissemination of promising practices to focusing on improving adoption rates. Teaching-track instructors such as lecturers with potential for security of employment (LPSOE) and lecturers with security of employment (LSOE) have adopted promising practices more than other instructors. Interview data show that LPSOEs are also effective at disseminating promising practices to their peers, but opinion leaders (influential faculty in a department) are necessary to promote adoption of promising practices by higher ranking instructors. However, hiring more LPSOEs or opinion leaders will not be enough to shift instructional practices. Variations in the adoption of promising practices by instructors and across departments show that any reform strategy needs to be systematic and take into consideration how information is shared through communication channels, the adoption decision-making process by potential adopters, and the contextual barriers and drivers of adoption. Additionally, the strategy should be designed with multiple stages, with each stage given time for changes to have an effect. Taking a one-size fits all approach to STEM education reform will not work and may only perpetuate the cycle of non-adoption and continued use of teacher-centered instructional practices. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]