Spectrum utilization, energy consumption, and cost efficiency are three key performance metrics that should be jointly investigated in developing a sustainable 5G system. Advanced spectrum sharing can enhance both the spectral efficiency and energy efficiency in a cost-effective manner, which is expected to perform much better than conventional networks. In this article, we survey cognitive and cooperative spectrum sharing, and classify a multi-level spectrum exploitation, coordination, and utilization framework from both technical and economic perspectives. We specifically concentrate on spectrum trading and leasing, spectrum mobility, relaying, routing, and harvesting. Finally, a spectrum flowing scheme is proposed for 5G cognitive heterogeneous cellular networks, which improves both spectral and energy efficiency.