The “active image” refers to the operative nature of images, thus capturing the vast array of “actions” that images perform. This volume features essays that present a new approach to image theory. It explores the many ways images become active in architecture and engineering design processes and how, in the age of computer-based modeling, images play an indispensable role. The contributors examine different types of images, be they pictures, sketches, renderings, maps, plans, and photographs; be they analog or digital, planar or three-dimensional, ephemeral, realistic or imaginary. Their essays investigate how images serve as means of representing, as tools for thinking and reasoning, as ways of imagining the inexistent, as means of communicating and conveying information and how images may also perform functions and have an agency in their own. The essays discuss the role of images from the perspective of philosophy, theory and history of architecture, history of science, media theory, cognitive sciences, design studies, and visual studies, offering a multidisciplinary approach to imagery and showing the various methodologies and interpretations in current research. In addition, they offer valuable insight to better understand how images operate and function in the arts and sciences in general.