An efficient (un)marshaling framework is presented. It is designed for distributed applications implemented in languages such as C++. A marshaler/unmarshaler pair converts arbitrary \emph{structured} data between its host and network representations. This technology can also be used for persistent storage. Our framework simplifies the design of efficient and flexible marshalers. The network latency is reduced by concurrent execution of (un)marshaling and network operations. The framework is actually used in Mozart, a distributed programming system that implements Oz, a multi-paradigm concurrent language. Mozart, including the implementation of the framework, is available at http://www.mozart-oz.org.