This paper is a review of some of the major applications of finite-state transducers in Natural Language Processing ranging from morphological analysis to finite-state parsing. The analysis and generation of inflected word forms can be performed efficiently by means of lexical transducers. Such transducers can be compiled using an extended regular expression calculus with restriction and replacement operators. These operators facilitate the description of complex linguistic phenomena involving morphological alternations and syntactic patterns. Because regular languages and relations can be encoded as finite-automata, new languages and relations can be derived from them directly by the finitestate calculus. This is a fundamental advantage over higher-level linguistic formalisms.