The book is based on the material originally developed for the course on Virtual Reality, which the author was teaching at Tampere University of Technology, as well as course on Virtual Environments that the author had prepared for the University for Advancing Studies at Tempe, Arizona. This original purpose has influenced the structure of this book as well as the depth to which we explore the presented concepts. Therefore, our intention in this book is to give an introduction into the important issues regarding a series of related concepts of Virtual Reality, Augmented Reality, and Virtual Environments. We do not attempt to go into any of these issues in depth but rather outline general principles and discuss them in a sense broad enough to provide sufficient foundations for a further study. In other words, we aim to provide a set of keywords to the reader in order give him a good starting point from which he could go on and explore any of these issues in detail.