A fundamental problem in wireless networking is efficient spectrum sharing. In this paper we study this problem in the context of decentralized multi-user frequency adaptation, with the objective of designing protocols that are efficient, agile, robust, and incentive-compatible. Our approach is based on the theory of congestion games, a class of games that models the competition for resources among multiple selfish players. In a congestion game, when a player unilaterally switches her strategy, the change in her own payoff is the same as the change in a global objective known as the potential function. Hence any sequence of unilateral improvements results in a pure strategy Nash equilibrium. In other words, the game is such that selfish behaviors collectively result in a socially desirable outcome. Motivated by the attractive properties of congestion games, this paper sets out to understand how this framework can be used to construct efficient spectrum sharing protocols. The key challenge in casting spectrum sharing as a congestion game lies in the proper definition of resources. Simply treating wireless channels as resources fails to capture the effect of spatial reuse. We first show how to reformulate two existing distributed spectrum sharing protocols as congestion games. Such reformulation is done by introducing virtual resources that model pair-wise interference. We then provide a new formulation by treating frequency-space blocks as resources. We use this formulation to construct practical protocols for spectrum sharing between multiple base stations/access points. Different implementation methods based on different signaling assumptions are discussed. We further demonstrate that the proposed approach can be readily extended in several aspects, including the modeling of channel bundling and fractional frequency reuse.