The underground economy represents a significant percentage of the Spanish economy, and the employment of undocumented migrants constitutes a large share of its supply of labour. This fact has functioned as an important pulling effect for the arrival of migrants into Spain over the last two decades, but especially during the last five years. Within their general strategy to fight against the underground economy, Spanish authorities have periodically used regularisation of undocumented migrants as a mechanism for emerging the irregular employment. In this vein, a series of schemes have been implemented with this objective over the last two decades. In this paper we review the importance of those processes for the transformation of Spain into a country of immigration, while stating the relatively limited efficiency of these mechanisms when applied alone to fight against a complex and multifaceted phenomenon like that of the underground economy.