In a sample of 323 engineering students, structural equation modeling was used to test hypothesized relationships between beliefs about the nature of knowledge in science, beliefs about the process of justification for knowing in science, and argumentative reasoning about an ill-structured social-scientific issue. Beliefs about justification for knowing were directly related to argumentative reasoning, with beliefs in justification by school-based authority being a negative predictor and beliefs in justification by research-based authority being a positive predictor of students' evidence- and knowledge-based reasoning, and with beliefs in personal justification being a negative predictor of students' consideration of counterarguments. These beliefs about justification for knowing in science were, in turn, predicted by students' beliefs about the certainty and development of scientific knowledge. The discussion highlights the multileveled and multidimensional view of epistemic beliefs that follows from our investigation, as well as the role of such beliefs in students' argumentative reasoning.