Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A Voice User Interface (VUI) makes human interaction with computers possible through a voice/speech platform in order to initiate an automated service or process. The VUI is the interface to any speech application. Controlling a machine by simply talking to it was science fiction only a short time ago. Until recently, this area was considered to be artificial intelligence. However, with advances in technology, VUIs have become more commonplace, and people are taking advantage of the value that these hands-free, eyes-free interfaces provide in many situations. However, VUIs are not without their challenges. People have very little patience for a "machine that doesn''t understand". Therefore, there is little room for error: VUIs need to respond to input reliably, or they will be rejected and often ridiculed by their users. Designing a good VUI requires interdisciplinary talents of computer science, linguistics and human-factors psychology - all of which are skills that are expensive and hard to come by.