De Castro, F. Keywords: Nationality | Dual nationality | Mots clefs: Nationalité | ABSTRACT Federico de Castro understands by dual nationality, the two nationalities that two States decide by mutual agreement to attribute to their own nationals. In a part devoted on nationality, the author examines whether there is a crisis of nationality, studies the history of nationality and its modern significance, and distinguishes in practice between actual nationality and formal nationality. He then discusses the issue…