Among the people who concern themselves with translation, there are three groups — teachers of translation, translation critics and translators whose work has not yet found acceptability in the target culture — who have a more pressing need than others for a definition of translation. Various definitions of translation have been presented, but they all seem to have recognized shifts of expression as unavoidable by-products of translation. The difference between the definitions is one of limits of shifts of expression. There are two ways to go about defining the limits: to evaluate shifts of expression against the degree of correspondence with the source text or, alternatively, against actual translations that are acceptable in the target culture. Adopting the latter approach taken by Toury (1980), the writer recognizes two general types of shifts of expression, compulsory and optional, and proceeds to talk in some detail about various types of optional shifts occurring in Persian translations. Finally the writer touches on the question of norms as indicating and governing shifts of expression. The article was originally a lecture delivered at Faculty of Humanities, Ferdowsi University of Mashhad. The main purpose of the lecture was to introduce Toury’s approach to translation studies, and to show its relevance and utility in tackling the present crisis in three areas of translation criticism, teaching translation and the practice of translation.