Published: 27 May 2024 The chapter first reviews the relative contribution of net migration and natural increase to overall population growth and shows that they often, but not systematically, vary in opposite directions, according to levels of economic development. In the second part, the focus moves to the two-way relationship between international migration and the demographic transition in developing countries. On the one hand, migration impacts fertility choices through ideational remittances, which are values and models transferred by migrants to origin societies. On the other, demographic change impacts migration as delayed marriage and drops in fertility generate a dramatic shift in the profile of migrants. The third part examines how high immigration combined with low fertility in the post-transitional demography of the most advanced countries does not produce two populations, one of local and the other of foreign origin, but one new population composed of individuals presenting an almost infinite number of possible combinations in terms of origins.