Efforts to reduce CO2 emission regarded as a main reason of the green house effect are widely performed in Information and Communications Technology (ICT) industry. In particular, as fast growing mobile communications services consume more energy, there are wide efforts to increase energy efficiency in the area of Mobile Station (MS), radio Base Stations (BS), network controllers, and core networks. User's concern, however, is more focused on optimization of energy efficiency in MS with limited battery capacity, because MS consumes much energy for wide broadband data services in data-centric communications services with 4G technology rather than legacy voice-centric communications services. The key idea of power saving mechanism (PSM) in MS is to operate sleep-mode that the MS turns down main elements when there is no data to receive/transmit in order to save battery power. With legacy voice-centric communication services, traffic pattern is rather simple and well-known so that the sleep-mode of PSM is well operated. However, in 4G technology for various data-centric services power saving mechanism should be adaptive to changing traffic condition in order to achieve optimal energy efficiency. Thus, holistic perspective for power saving techniques is needed in consideration of characteristics of services and QoS constraints, multiple applications support, remained battery power, handover process, Multicast Broadcast Service (MBS) support, and so on. This paper surveys various power saving mechanisms proposed for IEEE 802.16e/m. We first present the basic operating mechanism of PSM and research issues for performance enhancement. Based on their limitations and potentials, we then review several proposals of PSM for IEEE 802.16m, an evolutional technology of IEEE 802.16e. It will be interesting for readers to observe that once PSM was standardized in IEEE 802.16e, many new ideas have been proposed to improve the performance of the original PSM, and some of them are survived and adopted to new standard technique in the advanced IEEE 802.16m. (c) 2012 Elsevier Ltd. All rights reserved.