Understanding Interaction explores the interaction between people and technology in the broader context of the relations between the human-made and the natural environments. It is not just about digital technologies - our computers, smartphones, the Internet - but all our technologies, such as mechanical, electrical, and electronic. Our ancestors started creating mechanical tools and shaping their environments millions of years ago, developing cultures and languages, which in turn influenced our evolution. Volume 1 looks into this deep history, starting from the tool-creating period (the longest and most influential on our physical and mental capacities) to the settlement period (agriculture, domestication, villages and cities, written language), the industrial period (science, engineering, reformation, and renaissance), and finally the communication period (mass media, digital technologies, and global networks). Volume 2 looks into humans in interaction - our physiology, anatomy, neurology, psychology, how we experience and influence the world, and how we (think we) think. From this transdisciplinary understanding, design approaches and frameworks are presented to potentially guide future developments and innovations.