Knowledge in digital form offers unprecedented access to information through the Internet but at the same time is subject to ever-greater restrictions through intellectual property legislation, overpatenting, licensing, overpricing, and lack of preservation. Looking at knowledge as a commons—as a shared resource—allows us to understand both its limitless possibilities and what threatens it. In Understanding Knowledge as a Commons, experts from a range of disciplines discuss the knowledge commons in the digital era—how to conceptualize it, protect it, and build it. Contributors consider the concept of the commons historically and offer an analytical framework for understanding knowledge as a shared social-ecological system. They look at ways to guard against enclosure of the knowledge commons, considering, among other topics, the role of research libraries, the advantages of making scholarly material available outside the academy, and the problem of disappearing Web pages. They discuss the role of intellectual property in a new knowledge commons, the open access movement (including possible funding models for scholarly publications), the development of associational commons, the application of a free/open source framework to scientific knowledge, and the effect on scholarly communication of collaborative communities within academia, and offer a case study of EconPort, an open access, open source digital library for students and researchers in microeconomics. The essays clarify critical issues that arise within these new types of commons—and offer guideposts for future theory and practice.