In Chapter 2 we address the definitional question in order to circumscribe the meaning ascribed to the category of radical left populism. The objective is to distinguish this political field from the various forms of populisms found in European countries. By examining the political variable (crisis of democracy and political participation) and the economic variable (consequences of the Great Recession), and focusing particular attention on the combined response to those changes, this chapter will analyze the functional dynamics of the parties of the populist radical left, proposing a possible taxonomy of the various declinations and actually existing examples.