The aim of this paper is to describe the linguistic function of modal adverbs in natural languages. First, I shall introduce the concept of operator, as it should be defined within the framework of GB of TGG. At LF an operator is a function which, from a specific syntactic position, modifies a set of properties, and this information is going to be directly relevant for semantic interpretation. After pointing out the formal properties of operators (distinct from predicates), I shall demonstrate that modal adverbs behave at LF as propositional operators. Section 3 is devoted to a characterization of the semantic lexical function of each modal operator in terms of various modality scales, which are conceived to be conceptual scales defined over different sets of possible worlds. © 1987.