The purpose of this paper is to analyze assimilation in Russian by the Underspecification theory and Optimality theory. Underspecification theory is rule-based that was proposed with feature minimization in underlying representation. In this theory. certain values for all features are supplied by redundancy rules. and underlying representations are simplified. In process of assimilation in Russian underlying segments (m. n. l. r. j) are underspecified for feature value of (voiced]. Optimality theory is constraint-based and output oriented that provide optimal candidates by constraints and their hierarchy. This theory successfully provides optimal representations in process of assimilation in Russian. But it is still difficult to explain exceptions of assimilation. involving the softening. This paper deals with assimilation within morphemes with underspecified segments in the frame of Optimality theory and proposes the base of under-specification of sonorants by LICENSING. Finally. this paper presents that underspecification is necessary for descriptive adequacy in Optimality theory