The thesis analyses the legal opinions delivered by the Consiglio del contenzioso diplomatico of the Italian ministry of foreign affairs concerning the treatment of Italian nationals living abroad. As a premise, it recollects the different decrees and other pieces of legislation concerning the institution, functioning, assignments of the Consiglio. Having said that, the thesis focus on the opinions delivered in relation with actual disputes over the violation of the international law norms providing protection to foreign national residing abroad (ie. aliens) against their State of residence. That protection was implemented through the intervention of the State of nationality, according to the Vattelian paradigm which gives rise to the doctrine of diplomatic protection. In some cases, the claim was preliminary barred on account of the (purported) loss of Italian citizenship by the part of the emigrants. Accordingly, these disputes turn into disputes of nationality: with both States considering an individual as their subject. On the other hand, when nationality is not an issue, the disputes focus on the existence (or the non-existence) of customary international law norms on the treatment of aliens, their scope and the process through which their violation may be acknowledged and sanctioned.