Web services are changing the way in which the World Wide Web is currently being used. The Web was created originally to support human-to-computer interactions with textual and graphical data. Today, people use the Web to read the latest news, buy consumer goods, search for information, and obtain stock quotes. However, the Web does not yet support effective computer-to-computer interactions between software applications of different enterprises. Web services can enable Internet-based software applications of different enterprises to interact with each other directly by providing application programs with the ability to invoke operations that otherwise would be invoked manually by a human through a browser. Web services can run not only on mainframe computers and server computers, but also on desktop computers and client handsets. Web services allow individuals and organizations to publish links to their software applications, just as they publish links to their Web pages. Web services solve the enterprise application integration (EAI) problem by enabling interaction among different applications within the same organization. They can also enable computer-to-computer applications of different organizations to interact without human intervention. For example, Web services can be used for reservation systems, order-tracking systems, and business supply chains. In the example shown in Figure 1, Company A (a customer) orders goods from Company B (a distributor). Company B checks the availability of the goods from Company C (a supplier), and then arranges for payment and shipping of the goods to Company A without human intervention. Performance, security, and reliability are factors that will limit the use of Web services unless those issues can be properly addressed. Nonetheless, the potential widespread use and benefits of Web services are very compelling. Web services allow disparate computing systems and applications to be coupled together, and they enable enterprises to streamline and automate their business processes.