Informal economic activity is one of the most challenging and controversial aspects of human behavior. On the one hand, it challenges personal morality on the micro level as well as the authority of the state on the macro one. On the other hand, it is controversial as it has negative as well as positive consequences. The phenomenon of informal economy is neither novel, nor exclusive to the market or the command economies and is deaply analysed in this volume. In four chapters, 21 articles develops the following topics: definition and identification of the unofficial economy; its shape under different economic systems; its organizational forms; consequences and income distribution.