Smartphone fever along with roaring mobile traffic pose great challenges for cellular networks to provide seamless wireless access to end users. Operators and vendors realize that new techniques are required to improve spectrum efficiency to meet the ever increasing user demand. In this article, we exploit the great opportunities provided by cognitive radio technology in conventional cellular networks. Specifically, we first present challenging issues including interference management, network coordination, and interworking between access networks in a tiered cognitive cellular network with both macrocells and small cells. Taking into consideration the different network characteristics of macrocells and small cells, we then propose an adaptive resource management framework to improve spectrum utilization efficiency and mitigate the co-channel interference between macrocell and small cell users. A game-theory-based approach to efficient power control has also been provided.