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This Thesis deals with an innovative approach to the design of ground-source heat pump systems (GSHP), based on performance optimization during the entire operational life. Both design and management strategies are taken into account in order to find the optimal level of exploitation of the ground source, minimizing a proper performance index. The proposed method takes into account all the macro-systems governing the energy balance of the GSHP, namely: building thermal energy loads, efficiencies of the heat pump unit and back-up systems, and thermal response of the ground source. For each of them, suitable simulation models are presented and discussed. A rigorous mathematical formulation of the optimal design problem is provided, together with a specific resolution technique. In this regard, we also propose a statistically based evaluation methodology in order to analyze the soundness of the results of the optimization procedure. The main results of the proposed design and optimization methodology are: thermal capacities of heat pump and back-up generators, length and number of ground heat exchangers and the optimal load share between GSHP and back-up systems (control strategy). If installation costs and energy prices are taken into account, investment figures are also an output. We show how a proper synergy among GSHP and back-up generators leads to notable energetic and economic benefits, ensuring higher energetic performances, lower installation costs, and a sustainable exploitation of the ground-source. The proposed methodology can be conveniently applied to numerous professional, political, economic, and research activities. In this Thesis, we present two case studies. The first one refers to a typical professional design case, showing both the energetic and economic benefits achievable through the illustrated procedure with respect to traditional design methods. The second one illustrates as the proposed methodology can be applied to investigate the technological room for improvement of GSHP technology: in other words, we figure out the subsystem on which technological development should be focused, the expected benefits and some hints about a possible strategy for research activities.