The employment relations of international organizations are governed by their constituent instruments, internal laws and practice, with no overarching legal regime governing international civil service as such. However, academics and international administrative tribunals refer to 'international civil service law', while the tribunals also apply the general principles of such law. There is uncertainty as to what the legal basis behind this is and whether this refers to a common legal regime applicable to international organizations. This contribution analyses the existence or the possibility of such a common legal regime. It introduces the notion of 'international civil service' as a legal institution and international civil service law as its legal regime. The author analyses the theoretical foundation and potential normative bases of this common legal regime, setting out an agenda for further research.