Using procedure call semantics for distributed systems is generally considered a bad idea, so systems should communicate in a more loosely coupled way--preferably by exchanging messages. Message-based communication results in a simple interaction model and, if used with message queues, temporal decoupling between sender and receiver. Defining a conversation policy--that is, a description of all legal conversations--is an important step in establishing service contracts between providers and consumers, but doing so can be a complicated task. Luckily, this problem hasn't escaped the various standards and specification committees, so we have several approaches from which to learn.