By adopting a multi-disciplinary approach, this book provides a comprehensive analysis of the legality of the use of autonomous weapons systems under international law. It examines different arguments presented by States, roboticists and scholars to demonstrate the challenges such systems will create for the laws of war. This study examines how technology of warfare seeks to increase the dissociation of risk and communication between weapons and their human operators. Furthermore, it explains how algorithms might give rise to 'errors' on the battlefield that cannot be directly attributed to human operators. Against this backdrop, Dr Seixas-Nunes examines three distinct legal frameworks: the distinction between the legality of weapons and the laws of targeting; different mechanisms of individual accountability and the importance of recovering the category of 'dolus eventualis' for programmers and technicians and, finally, State responsibility for violations of the laws of war caused by weapons' software errors.