To estimate and analyse the underground economy phenomenon, different methods and approaches have been provided in literature. However, such approaches are very often based on static and linear equilibrium models and seldom adopt simulation tools. This paper results from a research project conducted in the Sicily Region (Italy) aimed at investigating – through the System Dynamics methodology – main causes-and-effects relationships underlying the phenomenon of the “hidden workers” at both firm and self-employment level. By combining micro and macro-analysis, the authors present a preliminary causal loop diagram of the investigated phenomenon. Such feedback structure has been built through the support of managers and representatives of private and public organisations.