Tense, aspect, and modality are grammatical categories that occur as functional heads in clause structure. They are traditionally grouped together by virtue of their semantic cohesion and their frequent morphological clustering or fusion. Research in generative syntax has focused on describing the morphosyntactic encoding of features related to temporal meaning, and on accounting for the general properties of tenses that occur in natural language. The tense section outlines empirical generalizations and their generative syntactic description. The term 'aspect' refers to two different layers of temporal information in the predicate phrase: the classification of events according to their temporal properties (stative/non-stative, punctual/durative, telic/atelic); and grammatical aspect, a temporal framework within which the event is located or described. Modality and mood are relational categories that express a mode by which a proposition is anchored to the external context of evaluation.