The paper provides an overview of the growing literature on two-sided markets, defining them as markets where the structure of charges (fixed and variable) matters for the volume of transactions between end-users. It highlights that the failure of the Coase theorem is necessary but not sufficient for two-sidedness. The authors develop a model that integrates usage and membership externalities, unifying two previously distinct strands of literature. They derive new results on the optimal allocation of membership and usage charges when price setting or bargaining determines payments between end-users. The paper also discusses the implications of these findings for various industries, such as videogame platforms, software production, portals, TV networks, and payment card systems.The paper provides an overview of the growing literature on two-sided markets, defining them as markets where the structure of charges (fixed and variable) matters for the volume of transactions between end-users. It highlights that the failure of the Coase theorem is necessary but not sufficient for two-sidedness. The authors develop a model that integrates usage and membership externalities, unifying two previously distinct strands of literature. They derive new results on the optimal allocation of membership and usage charges when price setting or bargaining determines payments between end-users. The paper also discusses the implications of these findings for various industries, such as videogame platforms, software production, portals, TV networks, and payment card systems.