At the moment the underground economy of the Mezzogiorno is widely debated. Studies commissioned by Bruxelles and an official document of the European Union estimate that the percentage of the Italian income produced by the underground section of the economy is "above 20%". This finding looks largerly consistent with a new wave of field research carried out in the Italian South: evidence on non registered labor - together with production, profits, wages, number of enterprises and even exports - looks often much higher that the ones officially recorded. Finally, first hand witnesses of the Mezzogiorno scene - as small firm organizations and local trade unions - produced evidence which work in the same direction. This essay focusses on the "measurement problem" of the underground economy and proposes a tentative picture of it and a few measures of political economy which seem to emerge from the debate.