This paper investigates some semantic and syntactic properties of infinitival relative clauses in superlative expressions (“infinitival superlatives”). In the course of this investigation, more general theoretical issues pertaining to the syntax and semantics of relative clauses in general are addressed, as well as the semantics of superlatives and of tense. The semantics of infinitival superlatives is accounted for by the assumption that the superlative morpheme takes an event argument bound by the matrix tense. It is also observed that not all languages have infinitival superlatives, and that not all the languages that have them admit them in the same constructions. It is proposed that these cross-linguistic differences are due to a difference in the syntactic status of the superlative morpheme.