Decentralised coordination in multi-agent systems is typically achieved using communication. However, in many cases, communication is expensive to utilise because there is limited bandwidth, it may be dangerous to communicate, or communication may simply be unavailable at times. In this context, we argue for a rational approach to communication - if it has a cost, the agents should be able to calculate a value of communicating. By doing this, the agents- can balance the need to communicate with the cost of doing so. In this research, we present a novel model of rational communication, that uses reward shaping to value communications. and employ this valuation in decentralised POM DP policy generation. In this context, reward shaping is the process by which expectations over joint actions are adjusted based on how coordinated the agent team is. An empirical evaluation of the benefits of this approach is presented in two domains. First, in the context of an idealised benchmark problem, the multiagent Tiger problem, our method is shown to require significantly less communication (up to 30% fewer messages) and still achieves a 30% performance improvement over the current state of the art. Second, in the context of a larger-scale problem. RoboCupRescue, our method is shown to scale well, and operate without recourse to significant amounts of domain knowledge. Copyright © 2009, International Foundation for Autonomous Agents and Multiagent Systems.