Abstract
Energy has become the most expensive and critical resource for all kinds of human activities. At the same time, all areas of our lives strongly depend on Information and Communication Technologies (ICT). It is not surprising that energy efficiency has become an issue in developing and running ICT systems. This paper presents a survey of the optimization models developed in order to reduce energy consumption by ICT systems. Two main approaches are presented, showing the trade-off between energy consumption and quality of service (QoS).