This introductory talk will set the context for the day. It will take a look at how energy efficiency is the major challenge for systems developers, and will then provide an overview of a number of open source projects that demonstrate how the energy efficiency of the entire system can be significantly improved. Energy efficiency of systems - hardware and software - matters. For the smallest energy scavenging systems it is a matter of eking out the picowatts. For handheld consumer electronics it is battery life that is a key product differentiator. Even for mains powered consumer devices, energy efficiency affects utility bills. And for datacenters run by the Googles and Facebooks of this world, more efficient systems mean fewer new power stations need to be built. This dev room is dedicated to the whole subject of energy efficiency in computer systems. This introductory talk provides an overview of all the approaches being taken to address this issue, particularly looking at how free and open source hardware and software is taking a leading role. It will provide a guide to the remaining sessions of the day, including the hands-on workshop where participants will have the opportunity to work with energy measurement hardware for themselves.