This book concerns one of the paradigm examples of grammaticalization, the development of a definite article from a demonstrative determiner. Although standard written Finnish has no articles, the demonstrative 'se' is currently emerging as a definite article in spoken Finnish. This book describes and explains the developing use of 'se' based on a database consisting of spoken narratives from three different periods spanning the last one hundred years. The author proposes that the development from demonstrative to article has its roots in the way that speakers ordinarily use demonstratives in conversation, and provides an analysis of the use of 'se' and the two other Finnish demonstratives, 'tama' and 'tuo' in a corpus of multi-party conversations, showing that speakers of Finnish use demonstratives to focus attention on important referents and to express and negotiate access to them in the interactive context of ongoing talk, and not primarily to talk about how near or far referents are. The development of se into a general marker of identifiability is shown to be connected with both the focusing function of demonstratives as well as its use for referents which the speaker considers accessible to the addressee.