This thesis examines the historical development of the expletive Ã¾aÃ° in Icelandic, from the earliest texts to the present day. This development is set against the backdrop of Icelandic clause structure, with particular attention to verb-second, information structure and the left periphery. The study combines corpus linguistic data and quantitative techniques with theoretical analysis, conducted within Lexical Functional Grammar. I show that Icelandic underwent three syntactic developments in the period 1750-present and argue that these all reflect one overall change: the establishment of Ã¾aÃ° as a structural placeholder for the topic position (the clause-initial prefinite position). I claim that Ã¾aÃ° functions as a topic position placeholder in the earliest attested stage of Icelandic (1150-1350), but is restricted to a specific context: topicless subjectless constructions with a clausal object, where Ã¾aÃ° has cataphoric reference. The three changes in the period 1750-present represent the establishment of this topic position placeholder in new contexts: (1) Ã¾aÃ° generalises to all types of topicless subjectless construction, beyond those with a clausal object; (2) Ã¾aÃ° emerges in presentational constructions (which inherently lack a topic), out-competing the earlier expletive form Ã¾ar; (3) in cataphoric contexts with a clausal subject, Ã¾aÃ° begins to transition from subject to topic position placeholder. The majority of these contexts exhibit at least a short period in which Ã¾aÃ° â or alternatively Ã¾ar â behaves like a subject. Icelandic thus exhibits the emergence of a topic position placeholder expletive from an earlier subject-like element. This shift towards prefinite expletives, which sets Icelandic apart from e.g. Mainland Scandinavian, happens relatively late in the diachrony (1750-present). Moreover, the Icelandic development challenges the standard claim in the literature on Germanic expletives, which assumes that subject expletives emerge from prefinite expletives.