While it lacks a definite article, Mandarin makes a principled distinction between unique and anaphoric definites: unique definites are realized with a bare noun, and anaphoric definites are realized with a demonstrative, except in subject position. The following proposals account for these facts: (a) bare nouns achieve definite interpretations via a last-resort type-shifting operator ι, which has a unique definite meaning; (b) demonstratives can occur as anaphoric definites because they have a semantic argument beyond their nominal restriction that can be filled by an index; and (c) bare nominal subjects are topics. A principle called Index! requires that indexical expressions be used whenever possible. Mandarin is contrasted with Cantonese, which, like English, is shown to have access to an ambiguous definite article.