Cellular network technologies have traditionally evolved to meet the ever-increasing need for capacity and coverage. As a result, several new technologies, as well as a significant focus on small cells and heterogeneous networks (HetNets), have been introduced. The latter are characterized by a large number of base stations (BSs) of different types, with more to come. However, the energy consumed by these BSs not only represents significant amount of operational expenses, but is also mostly wasted due to a low energy efficiency. The traditional approach to addressing such inefficiency has been to improve the hardware components. Alternatively, better network planning and deployment strategies, adapting BS’ active periods according to the traffic demands, and improving the communication techniques have shown promising results in achieving energy consumption reduction. The objective of this project is to develop techniques for energy-efficient communications in HetNets. We focus on developing energy-efficient techniques to utilize carrier aggregation (CA) and reduce the energy consumption at both the radio access network (RAN) and the user equipment (UE). Particularly, we propose a new cross-carrier-aware discontinuous reception (DRX) mechanism to reduce the energy consumption at the UE, and novel load-balancing techniques for multi-scream CA (MSCA) to reduce the energy consumption at the RAN.