This study examined how student achievement data are used to guide the development of curriculum documents in public school districts and within a commercial curriculum supplier. Two research questions guided this study: (a) How do public school districts in Central Texas use formative and summative assessment data to inform the written curriculum, and (b) how do commercially produced curricular programs use formative and summative assessment data to inform the written curriculum? A qualitative multiple-case study included curriculum developers from public school districts and a commercial entity. Data included semistructured interviews with curriculum developers from each organization as well as an extensive document review from each entity. The data were coded according to first-level coding and pattern coding. These themes were then analyzed through pattern matching and cross-case analysis. The research revealed that formative and summative assessment data were used to guide the development of the written curriculum in terms of guiding vertical alignment, determining the scope and sequence of the curricular content, adding specificity to the curriculum documents, identifying and correcting curriculum gaps, guiding development of formative assessment, and adapting to state and national change. In addition, the organizations utilized available resources in curriculum development and created a culture of data-rich dialogue. Findings also revealed that the ability of curriculum developers to use assessment data to inform the written curriculum is impacted greatly by organizational size and capacity. Sustainability of organizations to maintain a comprehensive, aligned curriculum is influenced by the rate of change coming from the state and national level. In conclusion, districts need to develop or obtain a guaranteed and viable curriculum that is strategically planned, comprehensive and aligned, as well as shaped by assessment data. The research reinforced that how data are created, presented, and used is important. Data sources need to be valid and reliable and shared in a risk-free culture that allows educators to move beyond elementary data uses to use data to inform the written curriculum as an integral part of school improvement. / text