The Vyadiyaparibhasavrtti is the earliest Sanskrit work entirely devoted to discussing the metarules and underlying assumptions of Panini's formal grammar of Sanskrit (5th-4th century BC). As such, it has much to teach us about the early development of ideas in this difficult and technical area which spans Indology and the history of linguistics. While the author does not admit it explicitly, he borrows heavily from Patanjali's Mahabhasya, but often changes or disregards the original purpose of these borrowed arguments. The discussions in the Vyadiyaparibhasavrtti are, in turn, typical of those used by later commentators, and the present book traces the development of these lines of thought from Patanjali in the third century BC to Nagesa in the seventeenth century.