Diagrams are all around us. Be they in the form of a hand-drawn map that shows you how to get to a friend's new house, as instructive illustrations in science textbooks, or as parts of an architect's design brief. This book systematically investigates the various connections between diagrams and the mental structures that are formed when we reason with them. It does so chiefly through methods of eye movement research and by assuming a constructive perspective that combines questions and approaches of basic and applied research from both Artificial Intelligence and Spatial Cognition. The book presents behavioral and computational methods to investigate human spatial reasoning and problem solving and establishes novel theoretical and practical foundations for more cognitively adequate live human-computer collaboration. It will be of interest to readers from computing who would like to know more about reasoning with diagrams in human-computer interaction and to the application-curious cognitive scientist.