In this paper we propose a model of human sentence processing that is based on Optimality Theory (OT). In contrast to most other OT approaches to language processing, we use constraints from OT semantics rather than OT syntax to address on-line comprehension. We illustrate the workings of our model by investigating the processing of coordinated structures. The psycholinguistic evidence that is currently available suggests that the on-line comprehension of coordination is influenced by constraints from many different information sources: pragmatics, discourse semantics, lexical semantics, and syntax. The model we propose formalizes this cross-modular interaction of constraints, and yields concrete predictions with respect to both intermediate parsing preferences and final interpretations. Our ultimate aim is to develop a model of processing performance is that at the same time a fully functional model of linguistic competence.