We sometimes disagree not only about facts, but also about how best to acquire evidence or justified beliefs within the domain of facts that we disagree about. And sometimes we have no dispute-independent ways of settling what the best ways of acquiring evidence in these domains are. Following Michael Lynch, I call this phenomenon deep disagreement. In the paper, I outline various forms of deep disagreement, following but also in certain respects revising and expanding Lynch’s exposition in (2010, 2012). As (...) is well known, for the externalist about knowledge and epistemic justification deep disagreements may be nothing more than an unfortunate failure of communication. Yet, though he grants this, Lynch argues that deep disagreement points to a sort of practical problem. I agree. In my paper I propose a revised account of the sort of practical problem that deep disagreement may pose. In short, my claim is that deep disagreement may be a problem due to the role that shared factual beliefs may have in common decision-making. I discuss and assess various reactions to the problem of deep disagreement, including the one proposed by Lynch. I argue that none of the solutions discussed in the paper are satisfactory. (shrink)