You reach for the bowl with ‘sugar’ written on it only to discover, from the bad taste of your coffee, that it contained salt. Mundane experiences like these show that epistemic justification does not necessarily hold stable across possible changes of information. One can be justified in believing a proposition at a certain time (that the bowl contains sugar) and cease to be justified at a later time, as one enlarges one’s epistemic perspective (as one drinks a salty coffee). When (...) this happens, one’s justification has been defeated. An epistemic defeater, broadly speaking, is that in terms of which the defeat of justification proceeds. The notion of an epistemic defeater is mostly associated with J. Pollock’s work on reasoning and inference. Pollock has provided the canonical definition of an epistemic defeater and proposed an influential taxonomy in terms of the way different types of defeaters induce their characteristic effects. The notion of an epistemic defeater has acquired a broader meaning in current epistemology, and several distinctions have been introduced that have contributed to a more nuanced understanding of the phenomenon of defeat as well as to complement Pollock’s original taxonomy. The correlated notion of a propositional defeater, defined in terms of would-be defeat of justification, is appealed to by the proponents of the defeasibility theory in the parallel debate about knowledge to explain what prevents a justified true belief from constituting knowledge in Gettier cases. (shrink)