The presence of foreign judges on the Constitutional Courts of Bosnia-Herzegovina and Kosovo is due to the existence of ethnic conflicts and societal divisions which resulted in the internationalisation of their constitutions and centralised constitutional courts. This chapter examines whether and to what extent the presence of foreign judges on these constitutional courts has met the rationale of helping to end or pacify the ethnic conflicts. It compares the influence of foreign judges on adjudication and constitutional law, and their contribution to the legitimacy of the courts in the eyes of the public and political actors. Differences between the working conditions of each court, degree of judicial activism, and the distribution of legitimacy between the court itself and the foreign judges, arise in each context, but the analysis ultimately suggests that the appointment of foreign judges to resolve ethnic conflicts is a ‘mission impossible’.