As professional learning communities (PLCs) are proliferating as a form of teacher professional development, it is important to understand what "PLC work" is and how it impacts teacher learning. This article reports on secondary science and mathematics teachers' participation in PLCs and engagement in collaborative inquiry. The PLC provided a structure for coming together; their inquiry questions focused their attention on gaps between a shared vision for student learning and student achievement. Qualitative data from three in-depth cases are analyzed in three categories: (1) collective activities, (2) questions raised, and (3) knowledge generated. The cases show different trajectories of teachers' PLC work and reveal the difficulties teachers had in asking critical questions about their practices. (Contains 2 tables and 1 figure.)