In all OECD countries, immigrants are concentrated in certain areas, especially in the poorer neighbourhoods and outskirts of the large metropolitan cities. However, not all immigrant groups tend to concentrate to the same extent, and concentration is shaped by both geography and historical settlement patterns. The effects of this concentration on integration are complex. On the one hand, arrival in an area with high concentration is often associated with better initial employment prospects for immigrants. On the other hand, in the longer run, immigrant concentration tends to hamper host-country language acquisition and, in many cases, educational advancement for children of immigrants. Policies should thus not primarily focus on preventing migrant residential segregation, but rather on enhancing mobility out of those areas. More attention should be given notably to the quality and accessibility of housing for immigrants.