Information from eye movements can be exploited in human-computer interaction on various levels. Offline recordings of eye movements give valuable advice for the developer by revealing the users' viewing patterns, time spent on looking at objects, objects that are not looked at all, etc. Real-time eye movements can be used as an input, either as an intentional, explicit control method, or as an additional information source that does not require the user to change his or her natural gaze behavior. In both cases, many of the principles and challenges are the same. In this talk, I will give an overview of gaze input for HCI and discuss design issues that should be taken into account when implementing gaze-aware applications.