This article argues for an activity-based theory of genre knowledge. Drawing on empirical findings from case study research emphasizing "insider knowledge" and on structuration theory, activity theory, and rhetorical studies, the authors propose five general principles for genre theory: (a) Genres are dynamic forms that mediate between the unique features of individual contexts and the features that recur across contexts; (b) genre knowledge is embedded in communicative activities of daily and professional life and is thus a form of "situated cognition"; (c) genre knowledge embraces both form and content, including a sense of rhetorical appropriateness; (d) the use of genres simultaneously constitutes and reproduces social structures; and (e) genre conventions signal a discourse community's norms, epistemology, ideology, and social ontology.