Purpose: The purpose of this paper is to focus on professional learning communities (PLCs) run for and by teachers to achieve their ongoing professional development and greater pupil attainment. The paper examined principals' perceptions of how such PLCs influence teachers, teacher learning and school processes, and their own involvement in PLCs operating in their schools. Design/methodology/approach: In-depth semi-structured interviews were conducted with 17 of 97 principals from three of the four educational districts participating in a pilot programme to achieve in-service teacher professional development through supported PLCs. Findings: Most of the participating principals considered PLCs to influence not only the teacher-leader, but also PLC members and other teaching staff and processes. Principals perceived PLCs as influencing pedagogical processes for both teachers and students, as well as staff leadership processes. Principals reported facilitating the operation of PLCs in their schools by providing the necessary conditions or participating in PLC meetings. Research limitations/implications: Although nearly 20 per cent of principals involved in the pilot agreed to participate, it was not possible to ensure a representative sample. Self-selection bias cannot be ruled out. Most participants were interviewed individually, with a minority later interviewed as a group to obtain a deeper understanding. Thus, the research should be regarded as exploratory. Originality/value: The study provides a detailed description of how principals perceive and engage with PLCs. Its findings are relevant to international efforts to understand and leverage teacher PLCs for the purposes of teachers' professional development and pupil attainment.