Evidence-based teaching (EBT), such as active learning and formative assessment, benefits student learning but is not present in many college science classrooms. The choices faculty make about how to teach their science courses are influenced by their personal beliefs and motivations, as well as their departmental structures and institutional cultures. With data from 584 science, technology, engineering, and mathematics (STEM) faculty trained in EBT, we compare which of the following factors most relate to faculty's use of EBT: 1) faculty's personal motivations (e.g., teaching value, confidence, beliefs about intelligence); and 2) their experiences with their institutional teaching environments (e.g., departmental support, student enthusiasm). Faculty's perceived "supports" in their teaching environments (e.g., having supportive colleagues, being able to access curricular resources) were by far most predictive of their use of EBT. Faculty's personal motivations had little to no relationship when supports were included in these models. The effects were robust, even when controlling for faculty gender, minority status, and teaching experience. Much of the literature has focused on perceived "barriers" to EBT implementation (e.g., lack of time, constrained teaching space). The current data indicate that a focus on building supports for faculty may have the greatest impact on increasing the presence of EBT in college STEM courses.