This book explores the duty to investigate potential violations of the law during armed conflict, and does so under international humanitarian law (IHL), international human rights law (IHRL), and their interplay. Through a meticulous comparative legal analysis, it maps out the scope and contents of investigative obligations. On the basis of general international law, it also develops and applies a novel and more broadly applicable step-by-step methodology for resolving issues of interplay between both legal regimes. In doing so, this study clarifies the scope of application and contents of investigative obligations under both legal regimes, as well as for situations to which both apply. The book finds that the oft-heard narrative that to require States to conduct human rights investigations during armed conflict would be wholly unrealistic in light of the realities of hostilities, is unfounded and in need of revision.