Purpose: As state and federal accountability systems have increased demands for evidence of student achievement, the use of data to inform practice has become more prevalent. More research is needed to understand not only "what" organizational factors shape data-use efforts but also "how" these factors enable or constrain educators' use of data for instructional improvement. This article addresses this gap by examining how two types of education systems--school districts and charter management organizations (CMOs)--use data and allocate their organizational resources to this end. Methods: Data were collected from six secondary schools in two districts and two CMOs during the 2010-2011 school year. Over 70 interviews were conducted with teachers and school and system leaders. Patterns from within and across school systems are presented. Findings: Key contextual differences had a strong influence on data-use efforts: Accountability pressures shaped the patterns in data use, whereas other organizational conditions--structure and decision-making rights, size and growth trajectory, financial resources, and degree of regulation--restricted or facilitated the systems' mobilization of resources for these efforts. Implications: This study suggests that the school systems as a whole play a critical role in supporting schools and educators in using data, regardless of whether that system is district or charter. As this article is one of the first to offer a comparative look at data use between school districts and CMOs, it lays the groundwork for diffusion of promising practices across these systems for school and system leaders.