The protection of marine environment assumes a special place under UNCLOS. Part XII provides for general obligations to protect and preserve the environment as well as more specific provisions concerning the prevention, reduction and control of marine pollution, including those relating to different sources of pollution. At the same time, Part XII is intended to provide a general framework for the protection of the marine environment. This framework nature is demonstrated not only by the existence of many generally-worded provisions and open-ended obligations which need to be further elaborated but also in the existence of numerous reference to external rules and standards, international organisations and regional arrangements. This chapter examines the contribution of UNCLOS tribunals to clarifying the scope and content of the general obligation to protect and preserve the marine environment before moving to discussing the extent to which UNCLOS tribunals have shed light on the normative content of three key environmental obligations and principles in the context of the marine environment.