This paper outlines a default and challenge account of a priori warrant by unfolding the three stages of the epistemic dialectic in which such warrant comes to the fore. Among the virtues of this account is that it does not rely on controversial assumptions regarding non-experiential sources of warrant, like intellectual intuition, but instead relies on features of our epistemic practice, more precisely, its default and challenge structure. What distinguishes beliefs to which you are warranted a priori is not that their source of warrant resides in some intellectual faculty, but rather the characteristic ways in which these beliefs can be successfully defended against challenges. The paper ends in a discussion of whether a priori warranted beliefs are empirically indefeasible, arguing that it is misguided to demand such indefeasibility of a priori warranted beliefs since that demand is not made for other sources of warrant. The question that rather should be posed is whether beliefs for which a priori warrant is provided qualify as knowledge on a consistent basis, and this question can be given an affirmative answer even in the face of empirical defeasibility.