New media, new knowledge practices, and concepts point to the need for greater understanding of cognitive processes underlying knowledge acquisition and generation in open informational worlds. The authors of the articles in this special issue address cognitive and instructional challenges surrounding multiple document comprehension--a prerequisite of digital literacy and knowledge work in the information age. In this article, we raise issues of conceptual clarity regarding constructs such as "source" and "document", and discuss work in disciplinary literacy and knowledge-building communities to clarify uses of information resources to support knowledge creation. The discussion aims to (a) provide insights into the cognitive and social practices needed for students to manage, understand, apply, and create knowledge and (b) suggest research directions to advance these goals.