“Mare Liberum” was published by Hugo Grotius in March 1609. 400 years later, a panel discussion in Potsdam focused on the classical text as such and then dealt with the reception of “Mare Liberum” both in Grotius’ time and today. Furthermore, it was discussed how Grotius himself was influenced by Francisco de Vitoria. These three papers are included in the present publication; a fourth paper on the current problems of fighting piracy by the European Union was presented but is not included here because we wanted to preserve the historical approach of the publication.