78 Fordham Law Review 2799 (2010)The Internet is a semicommons. Private property in servers and network links coexists with a shared communications platform. This distinctive combination both explains the Internet's enormous success and illustrates some of its recurring problems.Building on Henry Smith's theory of the semicommons in the medieval open-field system, this essay explains how the dynamic interplay between private and common uses on the Internet enables it to facilitate worldwide sharing and collaboration without collapsing under the strain of misuse. It shows that key technical features of the Internet, such as its layering of protocols and the Web's division into distinct "sites," respond to the characteristic threats of strategic behavior in a semicommons. An extended case study of the Usenet distributed messaging system shows that not all semicommons on the Internet succeed; the continued success of the Internet depends on our ability to create strong online communities that can manage and defend the infrastructure on which they rely. Private and common both have essential roles to play in that task, a lesson recognized in David Post's and Jonathan Zittrain's recent books on the Internet.