Based on two Romanian household surveys, we analyse the structure of households’ income by sources: main job, secondary job, and hidden activities. After conceptual clarification and explanation of the methodology we used, we estimate the size of informal economy, analyse the relationship between variables related to different types of income, and explore the dynamics of the informal economy. We find that the main participants in the informal economy are the poor people: the survival motive is dominant in the Romanian informal economy. We estimate that both in September 1996 and in July 2003 the income from the informal economy amounted to about 1/4 of the total household income (23.6% in 1996 and 22.7% in 2003, respectively). Also, we estimate the share of income from the informal economy in the cases of various categories of population (defined according to the dimension of the official declared income per person in the household). The extension of our analysis to the entire year using the household population structure by deciles suggests that the informal economy has increased, on average, by about 2-2.5% over the period 1995-2002. Indeed, beside the actual level of income, the households’ involvement in informal activities is probably influenced by occupation, region, age, education, number of children and many other factors. However, certain conclusions could be outlined: a) People perceive taxation as the main cause of the underground economy; b) Separating the main motivations of operating in the informal sector in two groups, “subsistence” and “enterprise” respectively, the surveys suggest that the subsistence represented a relevant reason for the households’ decision to operate in the informal economy, including its underground segment; c) Informal activities supplied a “safety valve” within the surviving strategies adopted by the poorest households; d) Participation in informal economy seems to be not simply correlated with poverty: in the informal economy are involved poor people (having probably a low educational level), as well as rich persons, but their motivations are quite different. The former are practically “forced” to operate in the informal economy (the “subsistence” criterion), but the latter are “invited” to participate in it (the “enterprise” criterion). In both cases, at least during the first stages of transition to a free market system in Romania, the environment was propitious due to legislative incoherence, feeble penalty system in the cases of fraudulent activities, and existence of some accompanying elements of proper informal activity, such as corruption, bureaucracy, etc. However, the household’s behaviour related to the participation in informal economy is sometimes fundamentally different between the two extreme groups of population. This is why in this study we focused on a deeper investigation of the behavioural aspects of different groups of population related to the implication in the informal sector.