This paper studies the driving forces behind the considerable expansion of the underground economy during the late 1990s. I propose a novel explanation for this phenomenon: the sharp increase in market competition worldwide, which reduces prices and profits and drives firms into the shadow economy. Empirical evidence from a panel covering 42 countries from 1995 to 2000 shows that increased competition is indeed correlated with an expansion of the underground economy. The effect is weaker in high-income, high-tax, low-corruption countries that provide public services which make it worthwhile for firms to operate in the official economy despite growing competitive pressure.