The aim of this work is to describe the different ways languages express focus and to explain why languages use exactly these ways. I give a detailed account of focus in three languages: Hungarian, English and Italian. The work is based on the assumption that the following principle operates at the interface between the grammar and the conceptualintentional system of the mind. (1) Focus Interpretation: The focus of a clause is any syntactic constituent that contains the main stress of the intonational phrase corresponding to the clause. (Following Reinhart 1995:62) It is widely believed that languages may mark focus by a special word order only. I argue against this view in my discussion about Hungarian. In Hungarian, a special word order is used when a particular element is in focus. I argue that this special word order is intimately related to the way main stress is assigned in the language. Thus I establish a crucial link between two types of focus marking: one which uses a phonological marker, e.g. English, and one which marks focus by special word order, e.g. Hungarian, reducing the latter ultimately to phonology. Subsequently I describe the architecture of the grammar from the perspective of focus marking. I show that languages that mark focus by main stress may realise marked focus patterns in three dfferent ways. Furthermore, I show that these three ways are the only possible ways to express focus by main stress. There is no other possibility available. From a theoretical perspective this work intends to investigate the role of the different modules of the grammar, in particular of syntax and phonology and the mapping between the two, in the representation of certain pragmatic notions such as focus.