This paper assesses the history and significance of Principle 24 of the Rio Declaration, which in 1992 called upon States to respect international law providing protection for the environment in times of armed conflict and to cooperate in its further development. In particular, the paper explores how the key elements of the principle have influenced subsequent law- and policy-making processes led by institutions such as the International Committee of the Red Cross, the UN Environment Programme, and the International Law Commission. The paper argues that while Principle 24 does not contain specific normative prescriptions, it has translated over the years into a significant and vibrant international law standard. However, in the light of the gaps and shortcomings that continue to characterize the protection afforded to the environment under international humanitarian law, the paper emphasizes the need to develop a comprehensive multilateral convention on armed conflict and the environment, with the aim of bringing the vision of Principle 24 into completion.