The article draws attention to a parallel between the current system of international law and the medieval legal system. The common denominator is the informal understanding of law and the basic forms (sources) of law – custom and practice. This is just one example out of a possible variety of analogies between the two systems. Besides, a relationship between the international law and history can be seen as manifested e.g. in deciding historical court cases or in invoking historical interpretation of current problems such as piracy. Moreover, the history of doctrine of international law can be researched further. The historical analysis of international law offers therefore a broad range of interesting topics to cover.