This dissertation examines the syntax and semantics of clausal complements. It identifies semantic underpinnings for some syntactic properties of the arguments of propositional attitude verbs. The way clausal arguments compose with their embedding predicates is not uniform and semantic differences emerge from the syntactic context clausal arguments appear in. Three case studies are taken up: clausal arguments of nouns, dislocated clausal arguments (sentential subjects and topics), and infinitival complements with overt subjects (AcI constructions). Chapter Two assembles evidence to support Stowell’s (1981) claim that the clausal complements of nouns are modifiers. It is shown that the clausal complements of nouns behave like adjuncts in their ability to bleed condition C (Kuno 2004 and Jacobson 2003, and explored here further). The compositional strategy used to compose attitude nouns with their arguments, following Kratzer (2006), is shown to account for this behavior and to be commensurate with observations made by Grimshaw (1990). I then show how the modifier status of clausal complements of nouns is determined by the way in which nominals are formed from clause-taking verbs. Chapter Three examines another complementation strategy, found with fronted clauses. New data from binding is provided in support of Koster’s (1978) hypothesis that clauses do not move. Specifically, fronted clauses fail to show the effects of syntactic reconstruction. An analysis, making crucial use of de re attitude ascription, is offered to account for ‘apparent’ binding into fronted clauses. Chapter Four makes the case for enriching the meanings of clausal complements. By examining some new patterns with accusative with infinitive (AcI) constructions (such as I see him to be a fool), I argue for decomposing certain doxastic attitude verbs, putting the introduction of alternatives into the complement. Here too the role of de re attitude ascription is shown to play a crucial role. It is argued that AcI constructions involve de re attitude ascription, with added constraints (determined by the lexical content of the embedding verb) on the nature of the Acquaintance Relation (Kaplan 1968, Lewis 1979). Several predictions about the kinds of verbs that can participate in AcI are borne out.