Book excerpt: Oceans and Aquatic Ecosystems theme is a component of Encyclopedia of Natural Resources Policy and Management, in the global Encyclopedia of Life Support Systems (EOLSS), which is an integrated compendium of twenty one Encyclopedias. The theme guides the reader through various pathways followed by surface water on earth. It describes the dominant processes that govern how organisms interact with water and with each other, and how they in turn can modify water properties. This knowledge is important for humanity. Indeed, only by understanding our actions impacts upon water, and the animals and plants living in it, can we learn to exploit water, marine and fresh-water habitats and the living organisms, without destroying the resources on which our livelihood and very survival depend. The Theme on Oceans and Aquatic Ecosystems discusses matters of great relevance to our world such as: Freshwater Wetland Resources and Biology; Problems, Restoration and Conservation of Lakes and Rivers; Coastal Regions; The Oceans and Seas; Oceanic Islands These two volumes are aimed at the following five major target audiences: University and College students Educators, Professional practitioners, Research personnel and Policy analysts, managers, and decision makers and NGOs.