The reduction of operational energy consumption alone will not be sufficient to enable and maintain sustainability. We need a holistic approach that considers not only the energy consumption during the usage stage but the entire product life-cycle from production to usage to disposal/recycling. We presented various approaches that have been proposed to reduce energy usage in those technological areas. We also identified some future trends and a few challenges we still need to address in the future. Information and Communication technologies are strong catalysts that can help boost productivity and efficiency in many sectors of the economy. These benefits are currently being reaped at the expense of increasing emissions of CO2 and the continuous depletion of fossil-based fuels used to generate the required electrical power needed to power ICT equipment. In addition, we are witnessing increasing energy costs year by year. To address these systemic challenges, we need to continue to develop innovative, cost-effective, and energy-efficient solutions that can minimize the energy consumption of ICT technologies and exploit renewable energy sources wherever possible. This can be achieved if power-awareness is fully taken into consideration in the design, implementation, deployment, and maintenance of ICT technologies. It is worth pointing out that various energy policies and standards will also continue to play a significant role in optimizing energy-efficiency of ICT equipment as well as their usage by the consumers. We do not cover these areas in this work. A detailed discussion of the impact of these efforts on energy efficiency is a subject of further study