This report provides an in-depth analysis of the duty of States to determine the legality of new weapons, means and methods of warfare under Article 36 in the 1977 First Additional Protocol to the 1949 Geneva Conventions. The right for States to use weapons, and means and methods of warfare in armed conflict is not unlimited. In armed conflict, international law - in particular international humanitarian law - contains restrictions in the use of weapons, and means and methods of warfare. To ensure that these restrictions can be adhered to in armed conflict, Article 36 imposes a duty on States to determine whether the use of new weapons, and means and methods of warfare is in accordance with international law. The present report provides a comprehensive interpretation of the meaning and scope of the terms of Article 36. First, it clarifies the subject of an Article 36- decicion, or, in other words, what constitutes a new weapon, or means or method of warfare under Article 36. Thereafter, it explains the situations in which the duty to make an Article 36-decicion arises, that is, the meaning of the terms study, development, acquisition and adoption. Lastly, it clarifies the requirements for the decision itself. Such requirements include, for example, at what stage in the procurement process a decision should be made, and the areas of the international law that should be taken into account in the decision. The final chapter of the report contains a summary of the suggested interpretations, as well as suggestions for further research.