The notion of focus structure in this work refers to the distinction between categorical, thetic and identificational sentences. The central claim is that the syntactic representation of every sentence has to encode which of these types of focus structure is realized. This claim is discussed in great detail with respect to syntax, intonation and semantics within the framework of the Minimalist Program. It is shown that the incorporation of focus structure into syntax offers new perspectives for a solution of vexing problems in syntax and semantics. For example, fronting (preposing, 'topicalisation') is treated as a syntactic operation which clearly belongs to core grammar, i.e. is not optional or 'stylistic'; the semantic notion of quantifier raising is dispensed with in favour of a focus structural treatment of phenomena which gave rise to it. The book appeals to generative linguists and to functional linguists who do not believe in an unbridgeable gap between the formal and functional analysis of language.