The impact of attending Prekindergarten on students’ readiness for and academic achievement during the Kindergarten year was investigated using data collected by a large urban school district. This study found that there is a significant, immediate effect of attending Prekindergarten on student readiness for Kindergarten. This effect dissipated during the Kindergarten year, showing significant effects only on some subscales of standardized testing used to measure achievement throughout the year. By the 4^th quarter, these effects were not found in the sample. Socioeconomic status was not shown to have a significant moderating effect overall, but gender did have a significant moderating effect on measures of Kindergarten achievement during the school year. Additionally, a review of the body of empirical evidence gathered to date about the efficacy of Prekindergarten programs, especially for low SES and minority students was conducted, with implications and recommendations for policy makers. This review focused on the components of highly effective programs as a model for policy makers to follow.