This introduction gives a rough sketch of the context of Mare liberum's publication and the main arguments Grotius made in this pamphlet. It touches briefly on some of the latest arguments on Mare liberum and provides a survey of the contributions to this Commemmorative Issue. Moreover, it sets the stage for the contributions which elaborate on the fate of Grotian concepts - not so much by historically tracing these ideas over the past 400 years, but by offering an analysis of how similar concepts are used in international law today.