This article provides an overview of the approach, methodology, and key findings from a theory-based evaluation of the district-led instructional reform effort in San Diego City Schools, under the leadership of Alan Bersin and Anthony Alvarado, that began in 1998. Beginning with an analysis of the achievement trends in San Diego relative to other California urban districts during this period, we then examine the theory of action that guided the San Diego effort, including the focus on instructional change as the primary means to improve student achievement and on the role of site-based instructional leadership and teacher professional development as central strategies for moving instructional practice. The article outlines the study design and introduces the set of papers in this special issue of "JESPAR." Reviewing key findings and themes across the article, we conclude that although the reform demonstrated that instructional improvement at scale is possible, sustaining that reform may be more elusive. (Contains 3 figures and 5 footnotes.)