It is submitted in this article that transfrontier agreements (of a publiclaw character) between decentralized authorities can be considered as a kind of international administrative agreements. After investigation of the power to conclude international (administrative) agreements and their (assumed) binding legal force in public international law, the article concludes that transfrontier agreements between decentralized authorities are in principle of a national and not of an international public law character. This article is also focussed on (overall) legal bases for transfrontier cooperation between decentralized authorities. In this respect the European Outline Convention on Transfrontier Cooperation Between Territorial Communities or Authorities is important and will be discussed. The purpose of this convention, in force in several European states, is to provide a legal basis for transfrontier cooperation (of a public law character) between decentralized authorities. However in this matter states still consider additional norms necessary. In this respect two additional treaties have been concluded and will be discussed. The article concludes that these treaties not really are overall legal bases for the conclusion of transfrontier agreements between decentralized authorities.