Adyasavada is the key principle of Advaita Vedanta. Basic premise of this theory is Adhyasabhashya. Before interpreting the first brahmasutra (athat¯o brahmajijnasa (Br.Su. 1.1.1), Acharya Shankara submitted a valuable commentary known as Adhyasabhashya. The book presents a comprehensive study of Adhyasabhashya in the light of Panchapadika of Padmapada, Bhamati of Vacaspati Mishra, Vivarana of Prakashatma Yati, Ratnaprabha of Govindananda Yati and other Vedantic texts. The numerous curiosities associated with Adhyasabhashya have been resolved in the present book: What is the format of Adhyasabhashya? Without commentary on any sutra can Adhyasabhashya be considered as a commentary? Adhyasabhashya begins with sentence yushmadasmatpratyayagocara so Adhyasabhashya consists mangalacharana or not? What is the purpose of Adhyasabhashya? Adhyasa principle is the cornerstone of Shankaravedanta. This is the theory of error of Shankaravedanta. In the context of theory of error Shankara proposed the theory of Anirvacaniyakhyativada and refuted all prior theories of error. In this book the commentaries of Brahma-Sutra and Shariraka-Bhashya and definition, causes, types, and results of adhyasa have been analysed in detail. Although it is difficult to write something about complex and difficult theory of Adyasavada, an endeavour is being made to bring out the theory of Adyasavada for scholars through this book.