Event extraction involves automatically finding, within a text, instances of a specified type of event, and filling a data base with information about the participants and circumstances (date, place) of the event. These data bases can provide an alternative, to traditional text search engines for repeated, focused searches on a single topic.

Constructing an extraction system for a new event type requires identifying the linguistic patterns and classes of words which express the event. We consider the types of knowledge required and how this knowledge can be learned from text corpora with minimal supervision.