All but the most briefly used systems must evolve as their mission and roles change over time. Evolution in the context of large distributed systems is extraordinarily complex because of the difficulty of upgrading all components simultaneously, and the fact that such systems are often very sensitive to changes in the message formats that underlay their communication. Prior approaches to the problem of implementing changes in a deployed system have relied upon ad-hoc solutions or protocol negotiation to avoid message format mismatches. We present a novel approach that combines message meta-data and dynamic code generation to create a robust messaging system that naturally supports application evolution.