The problem of human-computer interaction can be viewed as two powerful information processors (human and computer) attempting to communicate with each other via a narrow-bandwidth, highly constrained interface (Tufte, 1989). To address it, we seek faster, more natural, and more convenient means for users and computers to exchange information. The user’s side is constrained by the nature of human communication organs and abilities; the computer’s is constrained only by input/output devices and interaction techniques that we can invent. Current technology has been stronger in the computer-to-user direction than the user-to-computer, hence today’s user-computer dialogues are rather one-sided, with the bandwidth from the computer to the user far greater than that from user to computer. Using eye movements as a user-to-computer communication medium can help redress this imbalance. This chapter describes the relevant characteristics of the human eye, eye-tracking technology, how to design interaction techniques that incorporate eye movements into the user-computer dialogue in a convenient and natural way, and the relationship between eye-movement interfaces and virtual environments. As with other areas of research and design in human-computer interaction, it is helpful to build on the equipment and skills humans have acquired through evolution and experience and search for ways to apply them to communicating with a computer. Direct manipulation interfaces have enjoyed great success largely because they draw on analogies to existing human skills (pointing, grabbing, moving objects in space), rather than trained behaviors. Similarly, we try to make use of natural eye movements in designing interaction techniques for the eye. Because eye movements are so different from conventional computer inputs, our overall approach in designing interaction techniques is, wherever possible, to obtain information from a user’s natural eye movements while viewing the screen, rather than requiring the user to make specific trained eye movements to actuate the system. This requires careful attention to issues of human design, as will any successful work in virtual environments. The goal is for human-computer interaction to start with studies of the characteristics of human communication channels and skills and then develop devices, interaction techniques, and interfaces that communicate effectively to and from those channels.