Engineering green software-intensive systems is critical in our drive towards a sustainable, smarter planet. The goal of green software engineering is to apply green principles to the design and operation of software-intensive systems. Green and self-greening software systems have tremendous potential to decrease energy consumption. Moreover, enterprise software can and should be re-thought to address sustainability issues using innovative business models, processes, and incentives. Monitoring and measuring the greenness of software is critical towards the notion of sustainable and green software. Demonstrating improvement is paramount for users to achieve and affect change. Thus, the theme of GREENS 2015 is Towards a Green Software Body of Knowledge. Software can contribute to decrease power consumption— become greener—by being more energy efficient—using fewer resources; or by making its supported processes more sustainable—decreasing the environmental impact of governments, companies, and individuals using software applications and services. While research results exist in measuring and controlling the level of greenness of hardware components, major research is needed to relate energy consumption of hardware to energy consumption of its executing software. With the proliferation of smart devices, the Internet of Things, and the Industrial Internet the world is increasingly being instrumented with sensors and thus we have a better chance to quantify energy consumption due to executing software. We need to create new software engineering methods that aid professionals in addressing sustainability issues and energy efficiency. We need to be able to reliably measure the level of greenness of software systems at development time and at execution-time. Lastly we need to demonstrate green improvement to decision makers, companies, and end-users about the energy performance of the software they buy or use, to be able to show how user experiences, working habits, or lifestyles can effectively change to decrease their energy footprint.