The purpose of this paper is to outline the fundamental grammaticalization and degrammat(icalizat)ion processes observable in the restructuring of the tense/aspect/mood systems of the West Iranian languages during their historical development. Their core aspectual systems will be presented as consisting of three categories: Imperfective, Perfective and Perfect. Special attention will be paid to the rise of the analytic Perfect in Middle Persian and its further development in Early New Persian and other West Iranian languages. It will be shown that the degrammation of the copu-la played a significant role in the formation of compound temporal (Perfect, Pluperfect) and modal categories (Evidential, Conjectural) in New Persian, Kurdish, Balochi and Tajik. The Evidential mode of New Persian is based on the analytic Perfect rafte ast ‘he is gone’ and it is found in all the three aspectual categories (Imperfective, Perfective and Perfect) and both voices. It is usually claimed that it developed in the Iranian languages probably under Turkic influence. We intend to address the contentious issue of syntactic borrowing in terms of language contact in another paper.