This chapter illustrates the concepts of “children of immigrants” and “second generation” as they have been conceived and are employed in the sociology of migration. We begin by providing a short overview of the main theoretical perspectives on the integration processes of children of immigrants, including hypothesis on ‘straight-line’ assimilation, ‘segmented’ assimilation and the European centered ‘comparative integration context theory’. Next, we offer a comprehensive description of the different operationalizations and empirical definitions of the second generation used in the literature. Finally, we leverage existing evidence coming from selected empirical studies to show how these definitions affect the examined integration outcomes of children of immigrants. The chapter concludes with a summary and an outlook on future research.