The metaphor of a boundary as that which separates ‘us’ from ‘the other’ is central in modern discussion of identity as constructed, yet it is also recognized that such boundaries both articulate power and are permeable. The model is readily applicable to the Greco-Roman world where kinship, history, language, customs, and the gods supposedly separated ‘us’ from barbarians, but also enabled interaction; Jews and Christians engaged in the same strategies. At the textual level it is the different ways in which boundaries are constructed, particularly using diet and sexuality, that invite attention. This may offer a way of addressing questions of unity and diversity, of Judaism versus Judaisms, and of how ‘Christianity’ emerges as separate from ‘Judaism’.