Roma' depicts an ethnic and a collective identity, and so individuals' self-ascription as 'Roma' is only credible if legitimised through descent. This chapter focuses on endogamous population groups that occupy a particular socioeconomic niche in diverse societies around the world, specialising in a mobile, family-based service economy that often features a flexible portfolio of trades. Such communities are sometimes regarded as having a 'contrast culture', one that is dependent both culturally and economically on sedentary society but which cultivates its own particular identifiers in the form of both external emblems and appearance, and internal practices. In the early 1980s, the Council of Europe included both Romanies and Sami under its definition of 'nomadic populations', while contemporary definitions of 'Roma' in European policy documents tend to view peripatetics as commercial rather than pastoral nomads. Finally, the chapter also presents an overview of the key concepts discussed in this book.