This monograph challenges the prevailing teleological narrative that European political parties are inevitably converging toward a single model. The introduction establishes a new theoretical framework for analyzing the “crisis of parties”, moving beyond static ideal types to focus on dynamic dimensions of organizational change. It proposes two novel analytical lenses: Party Organizational Variance, which measures the structural diversity of party forms to test the limits of environmental determinism, and the Party Disintermediation Index, which tracks the systematic dismantling of intermediate bureaucratic layers. By integrating longitudinal data from eleven European democracies between the late 1960s and 2010s, the study argues that parties are simultaneously diverging in structure—driven by strategic agency—while converging functionally toward a “curvilinear” logic of disintermediation. This framework aims to explain how the removal of the “middle level” creates a direct, plebiscitary link between leadership and atomized members, fundamentally reshaping the nature of democratic representation beyond the traditional mass party.