The International Law Association Study Group on the Conduct of Hostilities in the 21st Century was established in 2011 and held its first meeting in 2012. The Study Group has explored numerous issues arising from the relationship between international humanitarian law (IHL) and international human rights law in the conduct of military operations, technological challenges posed by new weapons systems, and the basic principles of IHL in the conduct of hostilities. In 2015, the Study Group established three working groups focusing on core issues within IHL in relation to the conduct of hostilities in modern warfare. These working group topics were military objectives, the principle of proportionality, and precautions in attacks. Each of these working group reports contributed to this final report, which the Study Group presented at the 77th International Law Association Conference in Johannesburg, South Africa on August 7–11, 2016.