The development of a school improvement plan (SIP) has become an integral part of many school reform efforts. However, there are almost no studies that empirically examine the effectiveness of SIPs. The few studies examining the planning activities of organizations have generally focused on the private sector and have not provided clear or consistent evidence that such planning is effective. Some studies have even suggested formal planning can lead to inflexible and myopic practices or may simply waste time and resources. This study explores the relationship between the quality of SIPs and school performance by examining a unique dataset from the Clark County School District, the fifth largest school district in the nation. The study finds that, even when controlling for a variety of factors, there is a strong and consistent association between the quality of school planning and overall student performance in math and reading. (Contains 20 notes and 7 tables.)