It is widely believed that the Middle Persian vowel system was quantitative (e.g, Salemann, 1930; Rastergueva, 1969; Windfuhr, 1979; Pisowicz, 1985). This vowel system changed over time to result in its current main dialects: Dari (spoken in Afghanistan), Modern Persian (spoken in Iran), and Tajik (spoken in Tajikistan). The goal of this paper is to account for the development of the Middle Persian vowel system to these three dialects. The framework within which I present my analysis is modified contrastive specification (Dresher, Piggott and Rice, 1994). In particular, I follow the view that contrastive specification is the result of ordering features into a contrastive hierarchy (Dresher, 2003a, 2003b, 2003c; Dresher and Xi, 2005). This paper is organized as follows: Section 2 contains background on the vowel systems of Dari, Modern Persian, and Tajik. Section 3 suggests historical changes in terms of contrastive features in the inventories of these dialects. Section 4 concludes.