Mobile Network Operators (MNOs) seek to extend their infrastructure by installing more Base Stations (BSs), in an effort to increase the network capacity and meet the pressing traffic demands. To fulfill the escalated demands, Heterogeneous Networks (HetNets) constitute the new trend of next generation networks. The deployed infrastructure implies a rise in the Capital and Operational Expenditures. This book, therefore, focuses on the investigation of energy efficient solutions that bring down the energy consumption and the network cost, as well. More specifically, it provides contributions to the field of BS switching off strategies for wireless macro BSs networks and HetNets of single and multiple MNOs by proposing mechanisms that enhance different aspects of the network performance. The BSs deactivation, the innovative trend of infrastructure sharing and the financially driven collaboration among the involved parties of the current and future networks promise significant improvements in terms of energy and cost savings.