The present paper shows an introduction to the eye-tracking methodology applied to the study of language processing. Specifically, we review how certain cognitive processes involved in language processing (mainly, lexical activation and sentence processing) can be reflected (and, hence, studied) through eye-movement patterns. Even though this method was originally used in reading tasks, in this paper we also analyse its application to the study of language processing in other domains, such as the comprehension and production of oral messages.