Interaction designers often espouse general principles for creating more usable and satisfying systems. This book shows that to make those principles work, the designer needs to conduct rigorous analysis of the details of interaction, within a coherent formal framework. Through a wide-ranging collection of interaction examples and clear expositions of the relevant mathematics, Thimbleby demonstrates that careful analysis can improve our interactions with digital devices of all kinds. The book will be of value to anyone who creates interfaces to computers and electronic devices, from the smallest to the most complex.