Introducing the present issue, this article shows how the various texts that compose it contribute to the general debates surrounding bilingualism. We review the series of existing definitions which sometimes risk dichotomizing cases which are in fact extremely diverse. “Bilingualism” itself as an object is questioned, as are the possibilities of elaborating different sorts of typologies and their connection with questions such as identity or inter-generational transmissions. Considered from the standpoint of an ecology of language, language contacts touch on theoretical questions in the linguistic as well as the social sciences and philosophy, namely on the problem of the irreducible “One”, which seems to have hampered all recognition of the plural. That notion allows us to shed light on the fact that, despite the knowledge accumulated on bilingualism as a worldwide phenomenon, bilinguals continue to appear exceptional to speakers as well as – not infrequently – to researchers.