Global warming and climate change trends call for urgent action to manage information and communication technologies in a sustainable manner by minimizing energy consumption and utilizing resources more efficiently. Distributed computing environments have become the de facto platforms for many applications. These systems bring a range of heterogeneous resources that should be able to function continuously and autonomously. However, distributed systems expend a lot of energy which raises a range of important research issues related to the use and virtualization of ICT resources in a way offers significant potential to contribute to the goal of what has been described as `green computing'. This paper will review some of the important questions related to the development of new algorithms and tools for energy-aware resource allocation for large-scale distributed systems enabling these systems to become environmentally friendly.