Interjections in everyday talk routinely function as pragmatic markers, initiating utterances and relating them to the foregoing interaction. In turn-initial position, one finds both primary interjections like oh and mhm and secondary interjections like wow and boy. Much of the interactional significance of primary interjections derives from their characteristic position as turn initiators, and much of their meaning in any particular case depends on their intonation contour. Particularly secondary interjections display a range of functions, first acting as parallel pragmatic markers, but also in functions beyond parallel markers, namely with typical discourse marker functions of signaling contrast, elaboration and transition. On the basis of several large corpora of English conversation, this paper seeks to demonstrate the open-ended nature of the class of interjections, which apparently accepts an unlimited number of new items. Despite their variability, the pragmatic functions of ever new interjections seem always to be clear to participants in the concrete context. Interjections represent a large, potentially infinitely extendable class of items, unlike the relatively circumscribed, closed classes of other pragmatic markers, and their pragmatic marker functions follow from their general status as expressions of shifts in cognitive states of various kinds. Thus, I argue in favor of considering interjections a sui generis class with recurrent pragmatic functions, and seek to explain their pragmatic characteristics as far as possible in universal terms.