Hidden, submerged, domestic, community, alternative - a number of terms for the "other half of the economy", which is often not quantified in national accounts despite accounting for a large part of income. The 16 essays contained in the book, constructing models and describing heterogeneous international situations, help to clarify and give substance to the informal economy phenomenon, which cannot be confined to a single definition. Its relevance varies depending on reference parameters and local contexts. The development of the family economy is seen in relation to the crisis of the welfare state or the malfunctions of the State, undeclared work is seen in relation to the economic crisis and higher-profit mechanisms. Alternative economies are considered a "wellbeing protest" and a new "work style" in advanced areas, such as Germany, Sweden and Canada. In Italy, as can be seen by the extensive bibliographical essay and by A. Bagnasco's introduction, attention has focused on phenomena such as the decentralisation of production, small-scale entrepreneurship and dual jobholding. In short, the informal economy comprises a complex and contradictory realty, not necessarily reflecting modernisation-tradition or development-underdevelopment dualities, but is rather the expression of an increasingly complex relationship between the economy and society, in which the boundaries between informal and formal are growing increasingly blurred.