After clarifying the notion and different areas of the (digital) "public domain" – specifically with reference to related terms such as public goods and (anti-)commons –, the paper engages in discussing literature on its relevance for society in general and economic innovation in particular. For one, the public domain is not only inherently relevant for contemporary concepts of originality and authorship but also a precondition for democratic dialogue and free speech. For another, the public domain contributes to and enables economic innovation by fostering competition and by providing the intellectual raw materials for recombinant innovation and growth. How effective these abstract potentials of the public domain are utilized depends on the respective public domain regulation. In discussing the issue of regulation, the paper first introduces three basic regulatory dimensions: the temporal dimension, mostly dealing with length of protection terms; the territorial dimension, addressing the issue of jurisdictional borders; and the scope of the public domain with regard to different fields of application. Based upon this general systematization, the paper distinguishes different regulatory modes and arenas in both copyright and patent law, thereby focusing private regulatory initiatives such as Creative Commons or Biological Innovation for Open Society (BiOS). In the last section, the paper presents open research questions and makes some preliminary suggestions for potential research strategies. Specifically, more empirical research on actual usage of public domain materials in (inter-)organizational innovation processes and comparative assessments of different regulatory regimes both within and across jurisdictions seem to be promising avenues for further research.