The article takes a thorough look at the national legal framework concerning registrations in the National Record of Undesirable Foreigners and the Schengen Information System (SIS) within the context of the Schengen Agreement (1985) and the common rules adopted by the participating States. It analyzes the nature and implications of the actual administrative act of registration for foreign nationals in light of the pertaining domestic case law. A critical approach of the harmonization of Greek legislation to the Schengen rules is also being attempted.