The concerns about the rising energy consumption of data centers and the need to improve their efficiency have been shared by a large part of the research community and IT industry. As a result, a myriad of techniques have been proposed to curb the energy consumed by information and telecommunications technology. This article surveys techniques and solutions proposed to improve the energy efficiency of key components of data centers, namely computing and network resources. Existing techniques to evaluate and model the energy consumed by these resources are discussed. The paper also describes hardware and software approaches for reducing the energy consumption of entire distributed systems. The main goal of the survey is to review the state of the art and foster research on schemes to make network and computing resources more energy efficient.