is book aims to clarify the current debates regarding the concept of proportionality in international humanitarian law (IHL). Almost all states involved in armed conicts recognize the principle of proportionality in IHL as a legal and ethical restraint on their military activities. Specically, it is broadly accepted that an attacking party cannot justify collateral damage to civilians if the harm caused is expected to be excessive with regard to the concrete military advantage gained. However, the precise meaning of this principle is frequently disputed. ere are debates over many issues, including the degree to which force- protection measures can justify extensive collateral damage; the obligation to employ accurate but expensive weaponry; and the impact of using voluntary and non- voluntary human shields. ere are also disagreements concerning the parameters that are supposed to be used in assessing such a case: What exactly is a “military advantage,” and what does it mean for the harm caused to “exceed” such an advantage? Controversy is especially rife regarding asymmetrical conicts. Many states, among them many democracies, are engaged in armed conicts against non- state actors, some of them terrorist organizations. In these situations, questions relating to the correct interpretation and eective implementation of the principle of proportionality have arisen, for example, in connection with targeted killings and military operations in civilian areas.