Public communication's normative task is to support the legitimacy of collective decisions. Theoretically, two challenges in particular have proved persistent: (1) defining the purpose of public communication under conditions of pluralism, and (2) defining the composition of the public sphere as communication becomes increasingly transnational. It is argued that shared definitions of these, among actors participating in public communication, are prerequisites for the democratic legitimacy of collective decisions. Achieving this is difficult, particularly because it remains unclear how practices of public communication relate to ideals such as participation, inclusion and public reason. In part these difficulties can be attributed to a lack of congruence between the way political theory and empirical social research frame questions about the public sphere. To deepen understanding of these challenges, this study asks how purpose and composition are defined in Persian-language transnational newswork. It also asks whether communicating actors enjoy any meaningful definitional agency. The study is designed to align these empirical results with normative questions about public communication so that they speak more fully to one another. An interview-based qualitative study of the way newsworkers who engage in transnational Persian broadcasting define the public sphere provides the setting for this research. Newsworkers are examined because, it is argued, they enjoy a privileged kind of agency over processes of public communication and play an important role in the public sphere. The results show that transnational newsworkers enjoy some definitional agency, and that both purpose and composition find multiple, sometimes overlapping, and sometimes incommensurable and contradictory definitions in newswork. Newsworkers define a polymorphous public sphere characterised by a plurality of communicative purposes and constituted of a multiplicity of groups with different political allegiances. Some aspects of their definitions resonate with deliberative or agonistic conceptions of the public sphere. Despite these resonances, there are some contradictions between the requirements normative theory makes for a unified single-purpose public sphere and the multiplicity of purposes and criteria for inclusion found in practices of public communication. It is argued that these can be addressed by reducing the fact/value dichotomy and by shifting attention from compositional questions about the public sphere to a greater emphasis on the efficacy of public communication. This thesis contributes to the analysis of transnational and pluralistic public spheres. Moreover, based on both conceptual and empirical analysis, it examines how practices of public communication relate to ideals of the public sphere, an issue that is neglected in the literature