Mobile terminals in fourth generation cellular heterogeneous access networks often undergo horizontal and vertical handovers. To choose the optimal network considering required Quality of Service and efficient energy consumption, network selection process is a primary issue for mobile clients. The thesis delivers a handover decision algorithm that selects the optimal network from the available ones while considering the tradeoff between performance and energy consumption. The proposed method is suitable for deciding amongst networks while running our everyday mobile services as the most important QoS criterias are weighted according to user preferences using fuzzy logic. Energy cost is defined by aggregating multiple parameters including battery level or the energy component of the service being used and connecting to a certain network. A practical simulation environment demonstrates how the proposed method work in our everyday life and verifies the expectations and the efficiency of the algorithm.