This book provides a descriptive and comprehensive introduction to energy efficient networking systems and communication technologies. Energy efficiency in communication and networking systems is increasingly attracting attention due to both financial and ecological reasons. The amount of power consumed by information and communication technologies (ICT) is speedily growing, as well as the energy bill of service providers. According to many recent researches, ICT single-handedly is accountable for a major chunk of energy consumption which varies from 2% to 10% of the world power utilization. Energy-efficiency is a feature that until recently was only considered for battery determined tools. Nowadays, we see energy-efficiency becoming a persistent issue that will need to be considered in all areas of technology from device technology to systems organization. This book is a compilation of latest research data on hardware design, architectures, protocols and algorithms. This will not only help in making communication devices more energy-efficient but also lead to a future with more infrastructures based on energy-saving technologies.