This paper focuses on Tarskis declarative semantics and their usefulness in the design of a modeling language. We introduce the principles behind Tarskis approach to semantics and explain what advantages this offers in the context of modeling languages. Using sentential logic we demonstrate the necessity and sufficiency of Tarskis semantics for effectively addressing several issues that arise in the design of modeling languages. We explain what role Tarskis semantics play in the organization of a modeling language. This role is compared to the analogous roles of denotational semantics and operational semantics. We show that in the context of a modeling language Tarskis semantics are complementary to the other two kinds of semantics. The paper is intended to assist modeling language researchers and designers, particularly in connection with the UML - a language that in its current form does not feature Tarskis declarative semantics.