Since the beginning of the 20th century, the wireless frequency spectrum has been carefully controlled by government regulators. In response to the recent advances in radio technology, the spectrum regulators have opened some parts of the available spectrum for unlicensed usage. In addition, they have reformed the traditional command and control regulation policies and have allowed more opportunistic transmissions over unused spectrum bandwidth in licensed bands, for certain times and locations. This paradigm shift can lead to a more flexible and efficient spectrum sharing in the near future. In this chapter, we address the problem of spectrum sharing between network operators and cognitive radios. Because of the dynamic nature of spectrum sharing, it is difficult to analyze and to provide sound spectrum management schemes. Several researchers rely on game theory that is an appropriate tool for modelling strategic interactions between rational decisionmakers (e.g., spectrum sharing in wireless networks). We present a selected set of works to highlight the usefulness of game theory in solving the main problems in this field.