The average size of the shadow economy in Greece, 1980–2020, is estimated at 28.5% of gross domestic product, with an average annual rate of decline 0.41%. The finding of a negative relationship between unemployment and the shadow economy, combined with the significant and positive effects of self-employment and social insurance contributions, shows that the underground economy, in Greece, is driven primarily by people at work and not in unemployment. The official and unofficial sectors of the economy are complements. The political system is also of particular importance as a determinant of the size of the shadow economy, mainly capturing institutional and social aspects.