Developing students’ source-based argument writing skills is a vital educational goal for the 21st-century information society. Consequently, researchers and educators continually seek ways to understand and improve students’ capacities for constructing and advancing arguments based on multiple documents, texts, or sources in a range of subject areas in secondary education. To contribute to these efforts, this dissertation explores the relations between students’ source-based argument writing and a factor increasingly seen as a predictor of how students reason and write with multiple sources—their epistemology—beliefs about knowledge, and how it is constructed. In three studies, I examined: 1) secondary students' source-based argument writing skills in history, 2) students’ epistemologies in this discipline, and 3) the relations between student writing and epistemology. Findings from all three studies will help researchers and educators better understand students’ source-based argument writing skills, their views about the nature and construction of knowledge, and how these are related.