This chapter focuses on the inner structure of so-called patients’ organizations in the case of rare diseases. It compares eight such organizations, matching six rare diseases, to examine how decision-making takes place, what entities make up the organizations, which stakeholders are represented, what are their mutual relationships, what is the temporal scope of collective action and what are the concrete achievements of the organization. Two main types of organization are distinguished: “pluralistic” organizations (that bring together a broad array of diverse stakeholders who are willing to work together) and “monistic” organizations (where a single category of stakeholders takes the lead). Their operations are very different and can be related to the governance of the organizations. This suggests that both the usual opposition between lay and expert stakeholder and the reference to the diseases’ characteristics could be less relevant to understand patients organizations than the diversity of the stakeholders participating in decision-making.