Many technical issues like speech recognition and signal processing have to be addressed for the design of speech communication systems. However, before designing a speech application starts, 3 key questions that have to be answered: who will use it, why and how often? Only if the designer is clear about these questions, he is able to provide a maximum of target group orientation. This book gives focuses on these high-level questions and gives a criteria when and how to design speech systems. After an introduction, the state of the art in modern voice user interfaces is displayed. The following parts of the book evolve criteria for designing and evaluating successful voice user interfaces. In the last chapter, outlines and trends of this fast growing area are displayed.