The aim of this dissertation is to describe the sphere of use of the Russian Instrumental case in written sources from the eighteenth century. The research is based on approximately 11,300 instances of the use of the Instrumental and almost 2,400 constructions with other cases, excerpted from documents of various genres and styles. The corpus includes texts written by forty eighteenth-century authors, and contains works of poetry and drama, literary prose, letters, memoirs and learned tracts. Previous studies of the Instrumental case have in the main dealt with the development of the system of its meanings in the Old Russian period, or else have described its condition in modern times. The present work attempts to systematise its most typical uses and to trace the changes in the function of the Instrumental that took place during the period when a national literary language was coming into being in Russia. The research is primarily focused on the competition between the Instrumental case and other means of expression of particular meanings. In particular it describes (with statistical data) the variation in case forms within the predicate, with the function of an object, and also of the agent in passive constructions. A detailed description is given of those meanings of the Instrumental which are known from the earliest period and still in active use in the eighteenth century, but nowadays perceived as archaic. The most important of these are the Instrumental of cause, and also various uses of the Instrumental without a preposition to indicate time or place.