The distinction between functional categories and lexical categories is at the heart of present-day grammatical theory, in theories on language acquisition, code-switching and aphasia. At the same time, it has become clear, however, that there are many lexical items for which it is less easy to decide whether they side with the lexical categories or the functional ones. This book deals with the grammatical behavior of such in- between-categories, which are referred to here as "semi-lexical categories".