In this paper, we propose and analyze the properties of a new class of games-the network congestion game (NCG), which is a generalization of the classical congestion game (CG). In a classical congestion game, multiple users share the same set of resources and a user’s payoff for using any resource is a function of the total number of users sharing it. This game enjoys some very appealing properties, including the existence of a pure strategy Nash equilibrium (NE) and that every improvement path is finite and leads to such a NE (also called the finite improvement property or FIP). On the other hand, it does not model well spectrum sharing and spatial reuse in a wireless network, where resources (interpreted as channels) may be reused without increasing congestion provided that users are located far away from each other. This motivates to study an extended form of the congestion game where a user’s payoff for using a channel is a function of the number of its interfering users sharing that channel, rather than the total number of users using the channel. This naturally leads to a network congestion game, whereby users are placed over a network (or a conflict graph). We study fundamental properties of a network congestion game; in particular, we seek to answer under what conditions on the underlying network this game possesses the FIP or a NE. We also discuss the implications of these results when applied to wireless spectrum sharing in a cognitive radio network.