This book discusses human–computer interaction (HCI) which is a multidisciplinary field of study which aims at developing and implementing  tools and techniques to attain an effective and efficient interaction between the humans (the users) and computers. In recent years, there is an increase of interest of HCI researchers and practitioners in the inclusion of gaze gestures which can greatly enhance the communication between the human user and the computer, as well as other more “physical” communication involving all what can be learned from movements of the human body, from face, hand, leg, foot, etc., to the whole body movement, even extending to the involvement of groups of agents, even society. These explicitly human-centric issues in the development, design, analysis, and implementation of the HCI systems are discussed in the book. A comprehensive state of the art is given complemented with original own proposals. As opposed to more traditional formal and IT based analyses, the discussion is here more focused on relevant research results from psychology and psychophysiology, and other soft, cognitive, etc., sciences. Remarks on the relevance of affective computing are also mentioned.