Web services are a great mechanism for business-to-business interaction, or for any crossing of boundaries such as technological, trust and security, deployment topology, organizational, geographical, or other boundaries. At the heart of web services is the ability to develop such services and invoke their methods with the same ease as with conventional components. The web services standard even provides for defining interfaces or logical abstract services. As explained in Chapter 1 and demonstrated throughout this book, separation of interface from implementation is a core principle of component-oriented programming and is essential for application extensibility and reuse. This appendix presents a set of simple steps for developing interface-based web services using Visual Studio 2005. The source code accompanying this book contains the code presented here and the complete CalculationServices solution that uses it.