We distinguish the trajectory of European social democracy parties through four periods of development.
— Period I 1889-1914: Classical-historical Marxist social democracy.
— Period II 1918-1939: Evolutionary Marxist social democracy.
— Period III 1945-1989: Revisionism and traditional non-Marxist social democracy.
— Period IV 1990-2010: New revisionism and new social democracy.
Initially, we observe that each period-moment of social democracy is distinguished from the next by key events that constitute turning points in the historical process. Between the 1st and 2nd periods, the First World War and the October Revolution intervene. Between the 2nd and 3rd periods, the Second World War intervenes. Between the 3rd and 4th periods, the collapse of the regimes of the so-called "real socialism" and the end of the "Cold War" intervene.
Beyond the ideological and political content of the European social democracy movement, we also identify a corresponding periodization in the organizational forms that socialist, social democratic, and labor parties take. This relates to the well-known typology from the theory of the party phenomenon, the cadre party, the mass party, the catch-all party, and the cartel party.
In the chapters concerning the Greek case, a stricto sensu comparative approach is attempted, highlighting and justifying similarities and differences among political phenomena that self-identify with the same ideological-political and organizational terms, acting in different environments, both temporal and spatial.
