Scholars have become increasingly interested in what is often referred to as the instructional guidance infrastructure (IGI). Research has identified the characteristics of infrastructures that make them more or less influential in guiding teachers' instruction, such as alignment, authority, and prescriptiveness. Although these are important, a key question is whether and how IGIs work to influence teachers' beliefs and practices. We use new institutional theory to theorize relations between IGIs and teachers' beliefs about how students learn mathematics in two local school systems undergoing instructional reform. Based on new institutional theory, we explore how the three pillars of institutions--regulative, normative, and cultural-cognitive--worked in interaction to shape teachers' beliefs. In doing so, we develop a more comprehensive framework for examining IGIs and how they operate.