During the Symposium held in Manhattan in 2004 coining the One Health approach, the role of environmental law was underlined. The IUCN Commission on Environmental Law, through its representative from Southeast Asia, insisted on the importance of biodiversity conservation and the protection of wildlife while massive culling measures were taken to counteract zoonotic diseases. In this chapter we will show how the development of the One Health approach has been historically favoured by environmental law, acknowledging the interactions between health and biodiversity. We will detail how it has spread into the multilateral environmental agreements in relation to biodiversity conservation and how the environmental protection arena has evolved quite independently from the health sector in implementing the One Health approach until UNEP joined the FAO-OIE (WOAH)-WHO forces and the input from the OHHLEP (One Health High Level Expert Panel) in that respect. We will conclude with examples of One Health implementation in relation to environmental law, whether they concern research projects or training, notably in Southeast Asia.