This chapter presents data from a 15-year, mixed-methods school improvement effort. The High Reliability Schools (HRS) reform made use of previous research on school effects and on High Reliability Organizations (HROs). HROs are organizations in various parts of our cultures that are required to operate successfully "the first time, every time." This is a requirement increasingly placed on our schools. The HRS reform was conducted in all 11 secondary schools in one Welsh (U.K.) Local Authority (LA). Data are provided on the Neath-Port Talbot (NPT) district along with case studies of two of NPT schools. Pre-reform data indicated that students in the LA were performing well below Welsh averages. Over a four-year intervention, NPT's students made gains that were nearly double the national average. Achievement data from five and 11 years post-intervention indicate that the relatively high-poverty schools in the LA have continued to rise to well above the national averages. Qualitative observations and interviews with the schools' and LA's leadership and student groups, combined with analyses of recent school inspectors' reports, indicate that schools are continuing to use HRS principles and are continuing their refinement of them in context. Implications for future school reforms and research are presented.