Cogeneration system (CHP) is one of the ways to save the energy and use the energy efficiently. When compared to separate fossil-fired generation of heat and electricity. CHP may result in a consistent energy conservation (usually ranging from 10% to 30%) while the avoided CO2 emissions are, as a first approximation, similar to the amount of energy saving. In terms of sustainability, one of the primary considerations is energy efficiency. Sustainable energy is considered as a kind of energy which is renewable and continuous, meaning that the use of such energy can potentially be kept up well into the future without causing harmful repercussions for future generations. In this study, environmental benefits and sustainability aspects of cogeneration systems and importance of those systems to the use of sustainable energy are underlined. To support this idea, first we have referred some scientific studies previously made on cogeneration systems and then we have used our own case study. The case study made on gas engined cogeneration system was applied for a hospital to show the sustainability aspects of cogeneration systems. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.