An ecosystem is a community of living organisms such as microbes, plants, and animals and non-living abiotic components such as sunlight, water, air, minerals. There are aquatic ecosystems such as wetlands, lakes, coral reefs, hydrothermal vents, and others. An ecosystem can be described as a functional unit wherein living entities interact amongst themselves and with their surroundings – the physical environment. The chapter provides insights into the structure of the ecosystem, transfer of energy through the food chain, nutrient cycle and the output through the degradation and energy loss. They have also discussed relationships like chains, processes, webs that are produced as a result of these energy flows inside the system and their inter-relationship.