This book is all about designing and developing service-oriented applications using WCF. This appendix presents my understanding of what service-orientation is all about, and what it means to put it in a concrete context. But to understand where the software industry is heading with service orientation, you should first appreciate where it came from, since almost nothing in this new methodology is a leap of thought, but rather, it is based on a gradual evolution that spanned decades. After a brief discussion of the history of software engineering and its overarching trend, the appendix defines service-oriented applications (as opposed to mere architecture), explains what services themselves are, and examines the benefits of the methodology. The appendix then presents the principles of service orientation and augments the abstract tenets with a few more practical and concrete points required by most applications.