Understanding Knowledge as a Commons proposes that knowledge should be understood as a shared resource requiring collective governance. Digital knowledge can circulate widely, but it can also be enclosed through intellectual property, pricing, licensing, overpatenting, platform restriction, technical fragility and disappearance. The iconic idea is the knowledge commons. A commons is not simply free material placed online. It is a social, legal, technical and institutional arrangement that allows a resource to be created, preserved, accessed, governed and renewed. This distinction is crucial. Without rules, care and infrastructure, openness can decay; without access, preservation and collective responsibility, knowledge becomes vulnerable to enclosure or loss. The volume extends commons theory from natural resources to scholarly communication, libraries, open access, free software, public domain, civic engagement and digital preservation. Its importance lies in balancing possibility and threat. Knowledge has a special capacity for sharing because use by one person does not necessarily subtract use by another. Yet this abundance is politically fragile. The book asks how societies can build institutions that protect shared knowledge while sustaining the labour, funding and infrastructures that make it possible.