Uncommon Schools is a nonprofit charter management organization that starts and manages public charter schools, primarily in traditionally underserved communities. As part of a 2016 grant awarded to Uncommon from the U.S. Department of Education's Charter Schools Program to support replication and expansion of its school model, we conducted a rigorous external evaluation of the network's effects on student outcomes and the key practices implemented in schools that are part of it. The study aimed to address gaps in the existing evidence base for Uncommon's impacts, focusing on the network's two largest regions. We used a rigorous propensity score matching approach to examine the impacts of Uncommon middle schools in Newark and a lottery-based experimental approach to examine the impacts of Uncommon middle schools in NYC. We found that Uncommon produced substantial positive effects on student achievement in math and English language arts in Newark and no discernable effects on student achievement in NYC.