This chapter analyses how international humanitarian law (IHL) addresses harmful speech uttered against civilians in armed conflict situations. It investigates some practical examples from the ongoing armed conflicts in Ukraine, Myanmar and Gaza. The chapter finds that, if interpreted in light of humanitarian and international criminal practice, and when applied concurrently with international human rights law, IHL provides an adequately strong legal basis for field workers, investigators and prosecutors to gather evidence, investigate and prosecute the most serious cases of harmful speech as war crimes. In doing so, the chapter focuses on three aspects: the centrality of the prohibition of threats; the sanctioning of harmful speech by the occupying power in occupied territory; and an assessment of some examples of State practice and/or opinio juris, the constituent elements of customary law, to ascertain whether there could be an emerging general prohibition of incitement to commit war crimes. The findings are significant especially in the current polarised political climate which is not conducive to multilateral negotiations to enact new treaties or amend existing ones. As such, the chapter contributes to academic and humanitarian discourses on harmful speech, thus addressing one of the current challenges of humanitarians in armed conflict situations: what the law on harmful speech is and which speech acts can and should be investigated as war crimes. It demonstrates that IHL is ultimately resilient enough to withstand this pressure.