In some languages, such as French and Austrian German, the perfect construction is the standard tense/aspect form used to report past-time events. In many other languages, including English, the perfect construction alternates with other past tense forms, such as the preterit past (English) or the imperfect (French and many other languages), and there is considerable crosslinguistic variation on the precise usage conditions and semantics associated with each type of past tense form. Many of these languages exhibit the have/be alternation in the formation of the perfect, using have with transitive and unergative verbs, and be with unaccusative verbs. Many other languages, including English, use have uniformly. In this article I will seek to identify the syntactic source of the past tense meaning associated with the perfect construction. Because of the problem posed by the cross-linguistic variation in perfect semantics mentioned above, it is perhaps foolhardy to seek a single answer to this question for all languages, and a comprehensive treatment would require a dissertationlength study. For this reason I will focus on the English perfect construction, though I will occasionally rely on comparative evidence, especially with regard to the have/be alternation, and I will suggest the possibility of parametric variation. Even by focusing on the English perfect, we cannot fully avoid the problem of semantic variation, since the perfect construction does not have a uniform semantics in all its uses; according to many accounts, there are at least two, and perhaps as many as five, different uses of the perfect, each with a different tense semantics. For example, Brugger and D’Angelo (1994) have argued that the so-called universal perfect does not convey past tense; this claim is based on a particular set of syntactic/semantic diagnostics for past that they use, and is supported by the fact that many other languages convey the semantics of the universal perfect by means of the present tense..