We examined how the phrases I don’t know, I dunno, and idk are used in spontaneously produced speech and writing. We compared functions to the related phrases totally, absolutely, sorta, and kinda. We assessed usage across modalities (face to face, instant messaging, audiovisual), goals (tasks versus casual chat), and relationships (friends versus strangers). We also assessed where the phenomena occurred in a sentence, what words co-occurred with the phenomena, and what functions the phenomena served in the conversations. Communicators use phenomena differently depending on modality, goals, and relationships. We found that I don’t know was used more often when people could access cues beyond the voice, and that both I don’t know and I dunno can perform a variety of pragmatic functions. In instant messaging, I don’t know has been lexicalized to idk, but idk does not have as many pragmatic functions as I don’t know and I dunno.