This book develops a construction of functional categories in human language and applies it to the analysis of a variety of syntactic facts in Modern Germanic and Romance languages, as well as the history of English and Modern English. The construction accounts for previously unnoticed aspects of the development of the functional category system in child language acquisition, and for aspects of the developmental lag in functional categories by children with specific language impairment. The book presents a novel, tightly integrated approach to problems in syntactic theory and psycholinguistics, which will also be of interest to specialists in Germanic and Romance languages and the history of English.