The Parameter of Aspect presents a unified theory of aspect within Universal Grammar. It provides an unusual combination of syntactic, semantic, and pragmatic approaches to a single domain, and gives detailed linguistic analysis of five languages with very different aspectual systems: English, French, Mandarin Chinese, Navajo and Russian. Extensive discussion of the linguistic evidence is complemented by a formal semantic treatment in the framework of Discourse Representation Theory, with an explicit procedure for computing aspectual meaning from syntactic surface structure. Among the theoretical innovations is a principled account of the interaction between viewpoint (perfective, imperfective) and situation type (state, event); a level of pragmatic analysis at which contrastive and inferred meanings are stated; a default analysis of sentences that are neither perfective nor imperfective in viewpoint.