This thesis focuses on the energy efficiency (EE) of relay-aided cellular networks, which is motivated by the ever-increasing need to support higher and higher data rates, while reducing the energy costs. Relaying is a beneficial tool for either increasing the reliability or the coverage area of a wireless network as a result of the reduced communication distances, albeit this might increase the energy consumption in practice. Our approach in this thesis was to study and model progressively more complex and more realistic cellular networks. By utilizing novel transmission protocol designs, we were able to formulate their associated EE resource allocation optimization problems. Thus, efficient tools can be employed for solving these problems to maximize the EE.