Background: In 2008, Grossman and McDonald called for the development of a common language of "core practices" for describing the practice of teaching. They argued that a shared language of practice would help teacher educators and teachers work together on practice and develop nuanced, complex, and shared conceptualizations of instruction. Purpose: This article interrogates the claim that core practices promote shared language development by investigating a practice-based professional development (PD) program on project-based learning (PBL). Within the PD program, we study how teachers' conceptualizations of high-quality teaching shifted and how this related to their use of core practices language. Research Design: This qualitative content analysis examines if and how teachers' conceptualizations of high-quality teaching and their use of core practices language shifted across pre- and post-PD interviews. Conclusions: After the PD program, teachers' conceptualizations of high-quality teaching had changed. However, despite this shift, teachers rarely used the language of core practices when describing their teaching. These findings raise questions about the relationships between core practices of teaching, professional language, and teachers' conceptualizations of teaching. The authors discuss these questions and offer a framework differentiating core practices from best practices. The framework compares divergent purposes for developing a shared language to describe the practice of teaching.