The purpose of this article is to identify how principals shape the adoption and implementation of professional learning communities. The study employed a sequential mixed-methods approach in which interviews, observations, and document analysis informed survey design. Teachers were surveyed in four elementary schools about the practices and supports they received in grade-level professional learning communities. Qualitative and survey data reveal that principals influence both what teachers undertake in professional learning communities and how well they carry out these activities. The study was limited to a small sample of only four schools in two districts and relied primarily on principal and teacher self-reports. More work is needed to explore the role of principals in a broader sample as well as the relationship between district and building leadership. Findings are discussed in terms of what actions principals can undertake to sustain meaningful professional communities. This article provides guidance on how principals can influence and support teacher collaboration.