The electricity demand of our increasingly digital economies is growing at an alarming rate. While data centre energy demand has received much attention, of greater cause for concern is the growing energy demand of billions of networked devices. This publication probes their hidden energy costs. Being connected 24/7 means these devices draw energy all the time even when in standby. Studies show that for some devices up to 80% of the energy consumption is used just to maintain a network connection. In 2013 such devices consumed more than 600 terawatt hours of electricity. Implementing best available technologies could reduce energy demand of network-enabled devices by up to 65%. Exploring policy and technology solutions, the book charts a path forward and identifies which stakeholders are well-placed take the lead in particular areas. An underlying message is that there is a need for co-operation across all parts of the information and communication technology value chain.