This volume is a collection of papers on model theory and its applications. The longest paper, ""Model Theory of Unitriangular Groups"" by O. V. Belegradek, forms a subtle general theory behind Maltsev's famous correspondence between rings and groups. This is the first published paper on the topic. Given the present model-theoretic interest in algebraic groups, Belegradek's work is of particular interest to logicians and algebraists. The rest of the collection consists of papers on various questions of model theory, mainly on stability theory. Contributors of this title are leading Russian researchers in the field.