The present study sets out to investigate the structures and functions of vague expressions in Persian. The data under scrutiny include a 15-hour corpus of informal conversations. The corpus reveals some unique vague expressions including ‘rhyming words,’ ‘replacing expressions,’ and ‘the affective completer.’ Furthermore, a classificatory model is proposed that can be used to analyze vague expressions according to moment-by-moment communicative needs. The model divides the functions fulfilled by vague expressions into cooperative and noncooperative ones. Cooperative functions are those functions that serve to provide the hearer with enough information, however vague it is. Noncooperative functions are, on the other hand, those functions that show the speaker's unwillingness to provide the hearer with enough information. Cooperative functions are further divided into stance-oriented and engagement-oriented ones. While stance-oriented functions are motivated by the speaker's own judgments, engagement-oriented ones reveal a more social orientation.