This paper examines the process of progressive adaptation of nationallegal systems to the “civil law” legal system. The steady and widespread process ofdrafting legal instruments for legislative purposes at international level in order to“unify” the private law of the national legal systems confirms this evolution. At thisregard, it is to be highlighted the activity conducted since many years by the InternationalInstitute for the Unification of Private Law.The same phenomenon is happening within international public law by its progressiveand steady “codification” through the multilateral international treaties.The process of progressive harmonisation and unification of national legal systemsimplies a basic choice for “civil law” instead of “common law” whose raison d’etreand purpose is to guarantee a higher legal certainty in the inter-individual legal relationseither of private or of public persons. The “codification” of private internationallaw has the purpose of harmonising national legal systems of States in orderto facilitate legal relations and international “circulation” of national legal acts.Yet, the “codification” of international public law raises some doubts because theattempt to create “positive law” or to “crystallize” legal relations between States atinternational level is not convincing as a matter of law and fact.In fact, interstate legal relations are highly variable due to the ever-changing underlyingpolitical and economic grounds. Concerning the means, forms and timings that characterize the adaptation of international public law to such grounds and relatedchanges is very different from the way States’ legal systems adapt to new requestscoming from national societies.The paper also examines the role played by case-law in the relationship between“civil law” legal systems and “common law” legal systems.