Starting from the premise that democracy is a complex social system, this thesis contends that by investigating the complexity of what a system is in general, we can locate the distinctiveness of what democratic systems are and could be. This questioning takes roots in the theoretical context of the systemic turn in democratic theory. From a critical reappraisal of social systems theory, I demonstrate that it provides crucial tools to envision the complexity of democratic systems, and offer an analytical and evaluative framework of dynamic and ever-evolving democratic systems within their broader societal environment. In articulating political functions, democratic practices and democratic norms, this systemic framework transcends theoretical models' boundaries (deliberative, representative, etc.), and enables to map existing political systems and to identify their context-specific democratic shortcomings. I argue that such a systemic approach to democracy opens the road for a crucial endeavor: the possibility and necessity for citizens to diagnose democratically the problems faced by their democratic system. This thesis thus calls for the development of a critical democratic agenda: the constant diagnosis/problematization of democratic shortcomings, by citizens themselves and through democratic processes (including democratic innovations). In short, I justify democratic diagnosis as a necessary function of democratic systems and develop from a systemic perspective an analytical and evaluative framework to work with. I present the democratic function of democratic diagnosis as a major challenge and opportunity for both contemporary democratic theory and existing democratic systems.