The New York City Department of Education has supported collaborative inquiry as a potentially powerful process for helping administrators and teachers use student data to improve instruction and raise student achievement. Beginning with a pilot project in 2006, teams of teachers have learned to work together to diagnose the needs of students who have not been successful in their classrooms and to develop strategies to improve their learning. Collaborative inquiry sits at the heart of the Department's larger Children First initiative and aims to help educators close the achievement gap in their schools. Each year New York City schools have engaged higher proportions of faculty in the inquiry work. The goal is at least 90% participation in collaborative inquiry. The purpose of this research report is to share lessons learned about the conditions, structures, relationships, and leadership practice that support teacher participation in inquiry. The report also presents perceived benefits of collaborative inquiry as reported by school leaders and teachers. Data come from site visits to 13 schools actively engaged in collaborative inquiry. The research team conducted 213 interviews with principals, assistant principals, instructional support staffs, and teachers participating in inquiry and 37 observations of inquiry team meetings.