Mood refers to the grammatical expression of a speaker’s attitude toward the event(s) described by a verb, in particular the degree of a speaker’s commitment to the reality of the event. Languages vary greatly in the range of modal concepts they distinguish grammatically and in the ways they express them. In many Indo-European languages, verbal inflections can distinguish certain mood categories, specifically indicative (declarations of fact) and subjunctive (less certainly factual). Others, like English, rely on a series of auxiliary verbs to express degrees of certainty or commitment to factuality. Irish uses both strategies, which will be discussed in this chapter.