Research on source use and plagiarism in second language writing in recent years has suggested that rather than being the result of intent to deceive or moral failure on the part of the student, unacknowledged textual borrowing can have a number of causes, including cultural, developmental, and educational factors. This thesis aims to further research in this field by considering the complex relationship between student understandings of plagiarism and the ways in which they use sources to construct knowledge in their texts. This case study of an undergraduate English for Academic Purposes class uses a mixed methods approach, combining surveys, qualitative analysis of reflective journal and interview data, and text analysis to examine the student knowledge of source use and plagiarism from a sociocultural perspective. This allows for a fuller understanding of the varied contexts in which writing takes place, and the impact these contexts have on the students' writing processes. Analysis of this data revealed that a majority of students had some awareness of both the nature and importance of plagiarism as an issue at the university, as well as an understanding of the reasons for the restrictions on textual borrowing, but that there was a variation in the extent and specificity of that knowledge. It also revealed that a number of the students were very aware of the cultural, institutional, and disciplinary contexts in which they were writing. However, examination of the text data revealed that even a nuanced understanding of plagiarism did not guarantee the students' abilities to use sources successfully in their writing, and that they employed a number of non-standard textual borrowing strategies in their work. The thesis uses cultural-historical activity theory to help explain the relationship between aspects of student knowledge and practice, and to situate source use in second language academic writing within its wider cultural, institutional and disciplinary contexts.