This volume articulates a theory of syntax in which argument structure is the level of representation that underlies the alternative syntactic frames of a predicate such as we find in active/passive alternations. It argues for treating argument structure as distinct, in both form and substance, from other levels of representation, particularly those that represent grammatical functions and surface constituency, and related to them by correspondence principles. The evidence for this theory includes the analysis of reflexivised constructions, case marking, and causative constructions in Romance, with special focus on Catalan. These analyses raise important problems for commonly accepted assumptions in theoretical frameworks such as GB; for example, for the hypothesis that predicates have a uniform phrase structure representation of their arguments. It is argued that predicates have a uniform representation of their arguments at argument structure, which map onto possibly alternative overt expressions through the mediation of grammatical functions.