Implementing effective integration policies requires evaluating the extent to which outcomes of immigrants and their offspring differ from those of a reference group. When differences exist, it is important to identify clearly the reasons why. An immigrant population’s composition reflects successive waves of migration of persons of different backgrounds and skills and varies widely within and across countries. A detailed presentation of the socio-economic characteristics of immigrants and their offspring and comparison with a reference group is prerequisites to any assessment of outcomes. Variations in distribution by age, educational attainment or other socio-demographic characteristics between the target and reference population can make simple comparisons of the two groups’ average outcomes difficult to interpret. In addition to these socio-demographic characteristics, it is important to examine (when the statistical information is available) special features of the immigrant population, such as their language skills, the place where their education has been completed, their access to information about labour market opportunities and knowledge of the employment and social services in the destination country. While some immigrants’ specific features may hamper their outcomes, this should not be the case for the children of immigrants born and educated in the host country. The children’s outcomes are sometimes considered the benchmark by which integration is judged.The purpose of this chapter is to define and describe the different population groups examined in this publication. Section focuses on the immigrant population and Section on the native-born children of immigrants, including a comparison of their separate socio-demographic characteristics with those of the reference population. Section focuses on immigrant households in terms of size and composition. Overall, throughout the publication, there are frequent references to such contextual data in order to highlight differences observed between target and reference populations.