The book offers you a detailed description of vision algorithms and core building-blocks of a video-based eye tracking system (parts I and II), including: automatic detection of human eyes in videos, modeling the eyes' structure and appearance, tracking of spatial eye coordinates, identifying pupil, iris, eyelids, and eye-corners locations, estimating, calibrating and tracking a person's gaze direction, as well as interpreting and reconstructing displayed worlds' images on the eye's cornea. Part III through VI of this book provide a detailed coverage of recent successful use of eye monitoring systems, either as an interaction input device or a reliable source of data, in advanced applications including automotive, military, medicine, information security and retrieval, typing and reading, online search, marketing, gaze-contingent displays, augmented and virtual reality, and video gaming. Part III is on ``Gaze-Based Interactions'' and contains three chapters. Part IV includes two chapters on applications of eye monitoring in military and security realms. Part V presents recent deployment of the technology in automotive and medicine. The last part is on "Eye Monitoring in Information Retrieval and Interface Design Assessment".