This chapter explores the definition of the notion of ‘family’ from an EU law perspective. The chapter first acknowledges the variable geometry of the family, and the absence of a uniform category of ‘family’ in EU legal norms. The chapter then shows that, despite the fragmentation of sources and the modulation of family circles, the way in which the EU characterises a person as a ‘family member’ obeys a form of logic and expresses a certain rationality. Borrowing from the work of Morgan and his notion of ‘doing family’, the chapter demonstrates that in addition to the de jure family members, other persons are counted as family members on the basis of them ‘behaving’ like family members. Barbou des Places concludes that ‘family members’ is a defined category of EU law: it designates the groups of people who are assumed to perform – or asked to prove that they do perform – different functions like education, care, protection and socialisation. It is subsequently emphasised that these roles are central because they contribute to a broader ambition, namely, participating in the cohesion of the whole of European society.