In this essay prepared for the University of Pennsylvania’s conference on Intellectual Property and the Common Law, I build upon my work on custom and intellectual property. I focus here on one important facet of the subject — how longstanding common law principles should inform our understanding of custom. The common law provides a number of lessons on how to appropriately limit the consideration of custom in intellectual property law and elsewhere. The essay begins by considering the traditional role of custom in the common law. Part II then examines several of the ways that courts have incorporated custom into copyright law, particularly in the context of determining fair use. Part III explores recent efforts to use custom to ameliorate the uncertainty of fair use and to limit copyright’s ever-expanding boundaries. Parts IV through VI criticize the unreflected reliance on custom and consider appropriate limits on custom’s role, taking into consideration the traditional common law limits on the use of custom. Finally, I suggest a number of useful insights (other than the provision of legal rules) that custom and the common law provide for copyright law. These insights include the need for the copyright system to have public support to function, the importance of consensus for copyright rulemaking, and the need for a coherent normative framework. The history of the use of custom in the context of real property also reveals that uses of copyrighted works that cannot be substituted for or that serve communal recreational purposes should be given a privileged status.