The grammatical subject of sentences in English is regularly but not invariably perceived as the sentence topic. Attempts to express this regularity as a rule of grammar are frustrated by the numerous cases in which there is no topic or some other referring expression is the topic. An alternative account is one in which sentence topic is inferred on the basis of properties along two linguistic dimensions. The first is syntactic markedness: the more marked the syntactic structure, the more strongly the subject or other salient noun phrase (NP) is perceived as the topic. The second scale involves the referential explicitness of a possible topic NP. The NP types most compatible with being sentence topics are the ones which identify the most clearly their intended referent. In both cases, the definition of topic is based on the paradigmatic opposition of elements in English, serving as a "context" for all utterances. More marked structures and most explicit NPs have a communicative function of identifying what the sentence is about, relevant to the processing of syntactic-semantic structures in discourse. (Author/HTH)