Pronouns without explicit noun phrase antecedents pose a problem for any theory of reference resolution. We report here on an empirical study of such pronouns in the Santa Barbara Corpus of Spoken American English, a corpus of spontaneous conversation. Analysis of 2,006 third person personal pronouns in fourteen transcripts indicates that 330 (16%) lack NP antecedents. Of these, the following three types are most common: (1) pronouns whose referents are associated with a recently evoked entity ('inferrables'); (2) pronouns that refer to a fact, proposition, event, activity, situation, or reason evoked by a previous non-nominal constituent; and (3) pleonastic pronouns. We focus on some problematic cases that could be analysed as either referring to entities introduced by or implied in previous discourse, or as non-referential, including pleonastics. Such cases include certain non-specific uses of they, possible subjects of truncated cleft sentences, and possible subjects of truncated extraposition constructions.