Cloud computing promises a new era of service delivery and deployment in such a way that every person can access any kind of service from anywhere, anytime, using any device having internet connection. Sustainability of Cloud computing is to be addressed in terms of environmental and economic aspects. In this book we explore the energy efficient approaches inside data centres from the site and IT infrastructure perspective incorporating Cloud networking from the access network technologies and network equipment point of view to give a comprehensive prospect toward achieving energy efficiency of Cloud computing. Traditional and Cloud data centres would be compared to figure out which one is more reliable. Virtualization as heart of energy efficient Cloud computing has been introduced briefly. Finally approaches for Cloud computing data centres at operating system and especially data centre level are presented and Green Cloud architecture as the most suitable green approach is described in details. In the experiment segment we modelled and simulated Facebook and studied the behaviour in terms of cost, performance and energy consumption to reach a most appropriate solution.