When navigating through the vast amount of information that we encounter in our everyday lives, we constantly make decisions concerning what information to read more closely and what information to disregard. Consideration of the relevance and trustworthiness of information sources facilitates those decisions. In the present study, we interviewed 25 upper-secondary students after they had selected documents to use in completing a task, and we asked them to justify their selections. A qualitative analysis of the interview data showed that students' justifications could be related to (1) content relevance, (2) source features, and (3) prior topic knowledge. The results demonstrate that students are able to evaluate content relevance and source features as part of their document selection strategies but that perceived prior knowledge seems to influence both selections and justifications.