AbstractOver the last two decades, international humanitarian law (IHL) has seen a stalling with regard to States’ willingness to adopt treaties or to be formally involved in the development of IHL. This raises the question of whether holding on to the doctrine of sources as laid down in Article 38 of the Statute of the International Court of Justice is the only way to meaningfully further develop IHL. Indeed, in recent years IHL instruments have often dispensed with certain formalities that are traditionally linked to (the formal sources of) international law; this phenomenon is also called “informal international law-making” (IIL). The present contribution will analyze IIL as an alternative way forward in light of the current “deadlock” caused by States’ unwillingness to conclude new IHL treaties or to recognize customary IHL. In this article, we will investigate and assess the opportunities, shortcomings and pitfalls offered by informality by looking into examples of IIL within IHL. More concretely, we will look into State practice in relation to (1) the Safe Schools Declaration, (2) the Tallinn Manual and Tallinn Manual 2.0, and (3) the Montreux Document. Most importantly, our findings will assess whether IIL can overcome one of its alleged main disadvantages: its lack of effectiveness.