Perceptions and judgments on plagiarism or acceptable use of source texts are contingent on one's interpretations and experiences in reading and writing academic texts in a specific disciplinary context. The lack of consensus on what is acceptable textual appropriation in student writing has led to the scholarship on perceptions of textual appropriation in second language writing. As differences in individual perceptions are vexing problems for students trying to make citation decisions and distractions for faculty evaluating student writing, we need to understand and make transparent what influences students' and instructors' views on good use of source texts. The five papers in this special issue contribute to the scholarship on the topic by examining students' and instructors' perceptions using various methodological approaches including quantitative analyses of textual features, text-based interviews, and case studies based on a variety of data sources. In this introductory paper we relate the five articles in the special issue to previous scholarship, discuss some teaching implications that derive from the research reported in them, and advance new directions for examining source use.