In this text, the underground economy is presented as a natural reaction of consumers and producers to the constraints and costs imposed by the government on exchanges. We provide a definition of the underground economy and discuss the causes and consequences of this phenomenon. We also present an analysis of some methods used by researchers to measure its importance. We finally insist on the necessity in the future to obtain new sources of information in order to better understand the factors that influence the decisions to work and to consume in the underground economy.