This chapter explores the application of a key principle of the law of armed conflict—proportionality—in the context of new and emerging weapons systems and methods of warfare. The relentless pursuit of new military technologies by States continues to yield expanding lists of technology-related issues for lawyers to consider in applying the law of armed conflict in complex battlespaces on land, on sea, in air, in space, and in cyberspace. Foremost among these issues is the challenge presented by the principle of proportionality, requiring military forces to refrain from causing excessive damage to civilians and civilian objects when attacking military objectives. New weapons systems in complex battlespaces continue to increasingly force lawyers and decision makers to revisit, re-evaluate, and struggle in new contexts with the “equitable balance between humanitarian requirements and the sad necessities of war.” Some technological developments may, however, also present opportunities for the principle of proportionality to achieve greater relevance to the conduct of armed conflicts and even contribute to improved compliance by States. To illustrate these challenges and opportunities, this chapter examines the application of the principle of proportionality in modern armed conflicts with respect to several critical yet still evolving military technologies: unmanned aerial vehicles, autonomous weapons systems, cyber capabilities, and outer space technologies.