Immigration has been a steady force acting on population and employment within countries throughout human history. Focusing on the last four decades, we show that the mix of immigrants to rich countries has been, overall, rather balanced between college and non-college educated. The growth of immigration has been driven by immigrants from non-rich countries. The economic impact of immigration on receiving economies needs to be understood by analyzing the specific skills brought by immigrants. The complementarity and substitutability between immigrants and natives in employment, and the response of receiving economies in terms of specialization and technological choices, are important when considering the general equilibrium effects of immigration. In the United States, a balanced composition of immigrants between college and noncollege educated, together with the adjustment of demand and technology, imply that general equilibrium effects on relative and absolute wages have been small.