This study is concerned with the categorial status of subordinating conjunctions and the internal and external structure of subordinate clauses. Starting out from the categorizations of subordinating conjunctions that prevail in recent generative linguistic theory, namely complementizers and prepositions, and from the division of syntactic categories into lexical and functional ones, the author investigates the lexical and grammatical properties of subordinating conjunctions which are held to account for both the distribution and the architecture of subordinate clauses. Central to this study is the relation between the category subordinating conjunction, the licensing of its projection and the licensing of its complement and specifier position.