The societies of countries in the OECD and European Union have been shaped by successive waves of immigration. Their scale and makeup vary widely and many integration outcomes are shaped by different socio-demographic factors, such as place of residence, age, gender, etc. To interpret those outcomes, understanding differences in immigrants’ socio-demographic characteristics across countries and with their native-born counterparts is a prerequisite.This chapter looks at the broad socio-demographic characteristics of immigrants and compares them with those of the native-born population. considers the size of the immigrant population and the proportion living in densely populated areas. The chapter then goes on to address gender and age (), followed by birth rates and rates of unions with spouses or partners of the same origin ().The rest of the publication will make constant references to this background data as it seeks to explain some of the disparities that affect immigrants. For further discussion of issues raised in each section, see the section entitled Data limitations at the end of the chapter.