For years, the emergence and development of underground economical activities have been widely discussed in newspapers political circles and above all within daily life. This was however ignored by most economists, allergic to what cannot be taught by official statistics. If there are systematic factors which cause a shift of economic activity from the official to the unofficial sector, all possible forms of such shifts should be taken into account. Contrary, to others we therefore deal with a broad definition of the unobserved sector: it includes all economic activities that are not captured by the current official measurement system whatever the reason: be it that activities are hidden from the statistical observer because are illegal or that the concept underlying the national accounts framework is too restrictive. The unobserved sector could be separated into the non-monetary household economy and the monetary underground economy.