This thesis analyses the legal regime for international responsibility for activities in outer space, and with that, its central provision under international space law: Article VI of the Outer Space Treaty. The topic is assessed from the angle of public international law, and interprets Article VI of the Outer Space Treaty as an integral part of the international legal system. The analysis takes recourse to international responsibility law and the law of treaties. The result is a legal assessment of Article VI of the Outer Space Treaty that extends to States’ obligations in respect of space activities carried out by their non-governmental entities – a perspective of particular relevance at a time of considerable changes in the space industry, referred to here as the modern space age.