This chapter debates the role of the International Law Commission in codifying and promoting the progressive development of climate law. The ILC’s work on the protection of the atmosphere has proven to be particularly controversial. For Peter Sand, the ILC’s involvement is an opportunity to develop an authoritative interpretation of the ill-understood norms of general international law applicable to climate change. For Géraud de Lassus St-Geniès, the ILC has nothing to add to a problem that is being dealt with through treaty and international negotiation—which moreover requires an expertise which the ILC does not have and is about the mitigation of climate change rather than the ‘protection of the atmosphere’.