Eye tracking means detecting the user's gaze direction. Interfaces operated through the eyes are of great help for people with severe disabilities, allowing them to use their gaze to identify, or even move, objects on the screen, as well as to write. But eye tracking can be also used to improve ordinary keyboard- and mouse-based interaction: several explicit-communication eye-based interfaces have been developed to date, and might become popular in the future if costs of eye trackers will sufficiently go down. Eye-tracking is also studied and applied in several contexts besides that of an input means for interfaces, and many applications can be found, for instance, in Psychology, Psychophysics, Neuroscience, Usability, and Advertising. In our research we consider eye tracking both for implementing explicit/implicit interfaces and as a helpful means for the evaluation of web sites, information presentation modes and visual interactions in general. For example, we have developed Eye-S, a system that allows input to be provided to the computer through a pure eye-based approach. Another project is e5Learning, an e-learning environment where eye tracking is used to observe user behavior, so as to adapt content presentation in real-time. Also, we are studying the effectiveness of existing and new RSVP (Rapid Serial Visual Presentation) image visualization methods, which involve strong eye activity.