Adopting a legal definition of informality, this paper studies the causes of informality in general and with a particular application to Mexico. It starts with a discussion on the definition and measures of informality. It then argues and provides evidence that informality is not only a reflection of under development but may also be the source of further economic retardation. Next, the paper analyzes informality’s main determinants, arguing that informality is not single-caused but results from the combination of poor public services, a burdensome regulatory regime, and weak monitoring and enforcement capacity by the state. This combination is especially explosive when the country suffers from low educational achievement and features demographic pressures and primary production structures. The paper then evaluates the empirical relevance of each determinant of informality to the specific case of Mexico.