First Published in 1997. This work studies two related phenomena in human language: the ability of verbs and other lexical items to describe how a situation (event or state) develops or holds in time (LEXICAL ASPECT) and the view some verbal auxiliaries and affixes present of the development or result of a situation at a given time (GRAMMATICAL ASPECT). Through this investigation the author seeks to reveal a formal situation structure represented by aspectual phenomena, a structure to which other linguistic elements make reference, particularly tense. This structure describes the semantics of aspect and provides a principled input to pragmatic aspectual interpretation.