Charter public schools began as the "research and development" alternative to traditional public schools over 20 years ago, but today they account for significant portions of student enrollment in many cities. In more than 190 districts across the country, charter school attendance makes up at least 10 percent of public school student enrollment, and in 17 districts, over 30 percent of public school students attend charter schools. As charter enrollment grows, the consequence of traditional districts and charter sectors operating in isolation or--more commonly--with hostility and distrust, falls squarely on the shoulders of students and families, who fall victim to byzantine enrollment systems and timelines, a lack of accountability around equity issues like discipline or special education, inequitable funding, or missed opportunities for schools and educators to share their best thinking, strategies, and practices. Increasingly, however, cities have chosen to forge partnerships to address these common challenges. This brief describes the landscape of district-charter collaboration and the evidence to date, based on a recent Center on Reinventing Public Education (CRPE) study, on the potential and limitations of one strategy that many cities have used to develop more collaborative relationships: sharing instructional practices across district and charter schools.