What Is This Book About? This is a book about Web Services. Web Services are still more like a movement than a mature technology. The movement is motivated by a vision of a semi-auto­ mated Web that can support long chains of interactions between autonomous agents. There are three important components to that vision. One is interoperabil­ ity: a service can have clients (agents) from any platform, in any language. Another is autonomy: an agent can discover the services it needs from their published descriptions that include both what the service can do and how it does it (the interfaces of available actions). The third is (semi) automatic code creation: one description can be used by a development framework to automate the creation of code for clients and by the services themselves. As of today, interoperability is close to full realization, with only occasional glitches; autonomy is a distant vision; but it still has problems. Interoperability has been achieved code creation is useful in part by using an XML-based high-level protocol (SOAP) for message exchanges between clients and services. As long as the client can produce messages in the right format, it doesn't matter what language they're written in or on what platform they run. The first three chapters of our book show how to write platform-independent Web Services clients in Javascript and Java running from within a browser (IE6 or Mozilla).