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Water management brings together several issues of great importance, such as public health, environmental protection, and economic development. The need to protect and conserve water for present and future generations and to ensure its collective use has triggered extraordinary legal evolutions over the past five decades in most countries, whether in tropical, desert, or temperate zones. Traditionally, water has been apprehended in terms of its economic or social utility or according to its various states – running water, groundwater, and rainwater. If the first criterion unifies the legal status of the resource, the second contributes to fragmenting it. The challenges of conservation and collective use have given rise to management approaches based on a more unitary conception of water and seeking to reconcile the individual and the collective. These challenges strengthened State control over its water resources. The control of the State over water resources implies limiting and supervising its appropriation when the public interest is at stake. The reinforced control of public authority over groundwater illustrates a general trend. Several countries have integrated groundwater into the public domain or the public trust. Once incorporated into these categories, the right to withdraw water becomes subject to an authorization, license or concession issued at the national or decentralized level. This evolution also opens an opportunity to rethink the role of the State, in particular, its responsibilities as guardian of water. When mobilized for the conservation of natural resources, the public trust questions the nature of the prerogatives of the State, and in particular, the point of knowing whether these prerogatives are more akin to duties than rights. The dynamic dimension of the public trust is illustrated through the possibility open to the public of holding the State responsible before the judge for the degradation of the resources incorporated into it. The public trust’s added value is that it consecrates the existence of a kind of property for the benefit of the public. This conception of the public trust is increasingly mobilized in the context of climate justice.