This study builds on a 2019 Fordham Institute report that examined the relationship between charter school enrollment share--the share of students in a community who enroll in a charter school--and the average achievement of all the students in that community, including those in traditional public schools. Like its predecessor, this report seeks to understand the systemic effects of charter schools as well as the potential for diminishing returns as their enrollment share increases. However, unlike the first report, which focused on school districts, this one focuses on metropolitan statistical areas (MSAs), which are an order of magnitude larger and typically encompass multiple school districts. Fordham's associate director of research, David Griffith, analyzed a decade of data on reading and math achievement at the metropolitan level, as well as nearly two decades of data on charter and traditional public school enrollment. The results are summarized in three findings: (1) On average, an increase in total charter school enrollment share is associated with a significant increase in the average math achievement of poor, Black, and Hispanic students, which is concentrated in larger metro areas; (2) On average, increases in Black and Hispanic charter school enrollment share are associated with sizable increases in the average math achievement of these student groups, especially in larger metro areas; and (3) On average, an increase in total charter school enrollment share is associated with a significant narrowing of a metro's racial and socioeconomic math achievement gaps. [Foreword written by Amber M. Northern and Michael J. Petrilli. For the 2019 report, "Rising Tide: Charter School Market Share and Student Achievement," see ED601200.]