This article discusses the combination of two professional development strategies--lesson study and the Sheltered Instruction Observation Protocol (SIOP)--which gave teachers in Freeport Public Schools, a district on Long Island, New York, new tools for improving instruction for ELLs in mainstream classrooms. The unique combination of the two models underscored the strengths of both in terms of teacher development and impact on student learning. As a result of this fusion, participating teachers collaboratively planned content-based lessons, observed each other teaching these lessons, discussed their observations, reflected on SIOP techniques and their impact on student learning, and prepared lesson study reports. The authors share the lessons learned from the implementation of the two professional development models linked by the common feature of engaging practicing teachers in collaborative inquiry to respond to the needs of ELLs.