Book excerpt: This study aims at developing a unified perspective on nonfiniteness, encompassing its morphological, syntactic and semantic aspects. It puts the emphasis on clause types distinct from standard infinitives (gerund clauses, Celtic verbo-nominal structures, Portuguese inflected infinitives, Latin dominant participle constructions) and takes advantage of the most recent developments in syntactic theory. The notions of defectiveness and completeness, the inheritance hypothesis, the labeling requirement, the syntactic definition of lexical categories, once combined together, appear to make accessible tighter and more elegant analyses than previous accounts.