INTRODUCTION Charter schools have been one of the most significant developments in U.S. education over the last 20 years. Since the first charter school program was introduced in Minnesota in 1991, 42 states and the District of Columbia have adopted charter school legislation. In the 2012-13 school year, roughly 6,000 charter schools were open nationally, enrolling more than 2.3 million students (National Alliance for Public Charter Schools, 2013). Th e growth of charter schools has continued steadily, with roughly twice as many students in attendance as listed in the 2008 edition of this volume. 1 Although only about 4 percent of students nationwide attend charter schools, these schools serve more than 10 percent of the public school population in over 100, mostly urban, school districts (National Alliance for Public Charter Schools, 2012).