Seeking to balance military necessity with humanitarian considerations, the Law of Armed Conflict (LOAC) restricts the manner in which force can be used during warfare and prohibits certain kinds of attacks. In order to protect civilians and civilian objects while pursuing attacks against combatants and military objectives, LOAC prescribes strict targeting rules which are based on the principles of distinction, proportionality and precautions. The investigation of civilian casualties that have occurred during armed conflict enforces (and reinforces) these targeting rules and serves a similar purpose—protecting the civilian population from the effects of military operations. The present article discusses the circumstances which give rise to a duty to investigate civilian casualties. It considers whether and to what extent a State whose armed forces caused civilian causalities during combat is required—under LOAC—to investigate these incidents. While the starting point is the case where a suspected war crime is involved, the article also deals with the question whether there is a duty to investigate incidents which indicate a LOAC violation that does not trigger individual criminal responsibility, namely a suspected failure to take feasible precautions in attack. It further considers whether an obligation to investigate arises in each and every case of civilian casualties, notwithstanding they are not necessarily unlawful under LOAC.