We have explored the effect of information structure on grammatical marking, presenting evidence from languages that treat topics specially in terms of case or agreement. Topicality is a relational property of a referent, determined by the speaker's assessment of its relative saliency, and cannot be “measured” in terms of inherent semantic features such as animacy or definiteness: topical referents are what propositions are construed to be about. Crucial to our analysis is the possibility for more than one sentence element to be topical. We distinguish between the primary topic and the secondary topic; both are topics, but the primary topic is more pragmatically prominent. Although there is no unique alignment between information-structure roles and grammatical functions, there are important cross-linguistic tendencies in the grammatical expression of primary and secondary topics: in particular, we have argued that while subjects are prototypical/canonical primary topics, objects tend to be associated with secondary topics.

In the simplest cases of apparently “optional” case- and adpositional marking and agreement, the factor determining the presence of marking is whether a sentence element is topical. In some languages, casemarking and agreement mark the topical status of any grammatical function, subjects as well as non-subjects. Other languages grammatically mark topicality for a range of non-subject NPs. Restrictions on marking in these cases have often been treated in syntactic terms, but we believe that some of these apparent syntactic restrictions may be better thought of as a consequence of independent constraints on how topics can be syntactically realised.