The no-harm principle is at the heart of the several international conventions focusing on the uses, allocation, management and protection of transboundary water resources. However, in the framework of these agreements, the meaning of “no-harm” remains rather vague. Through an analysis of six emblematic cases brought before the International Court of Justice and arbitration tribunals, we elucidate the various facets of this principle. In doing so, the paper identifies four facets. The first is characterized by concerns related to the protection of territorial integrity rather than those related to the protection of the environment. The second facet focuses on the principle of equitable and reasonable use of water, which testifies to the willingness to anticipate possible harms and to define conditions for cooperation between neighbouring countries. The third facet explores the use of three instruments and emphasizes their importance to clarify the very nature of harm: the conduct of environmental impact assessment, the consultation of local populations and the insurance of minimum environmental flows. The fourth facet develops a preventive perspective on harm by unravelling the duty to take appropriate measures to prevent and mitigate risks deriving from the obligations of notification and consultation.