This paper studies the main factors of economic immigration to Spain between 1994 and 2004. From the main theories of international migration movements, a panel data with information on fifteen countries less developed than Spain is analysed. Our results show that the stock of immigrants of the same country already living in Spain (network effect), the poverty level and the degree of urbanization of the country, differences in the unemployment rates, geographical distance and cultural affinities are determining factors of the Spanish international immigration.