This paper develops a non-relativist version of contextualism about knowledge. It is argued that a plausible contextualism must take into account three features of our practice of attributing knowledge: (1) knowledge-attributions follow a default-and-challenge pattern; (2) there are preconditions for a belief's enjoying the status of being justified by default (e.g. being orthodox); and (3) for an error-possibility to be a serious challenge, there has to be positive evidence that the possibility might be realized in the given situation. It is argued that standard "semantic" versions of contextualism (e.g. those of Lewis, Cohen, DeRose) fail to take these features into account, which makes them overly hospitable to the sceptic, and that Williams' version of contextualism, although incorporating (1), fails to do justice to (2) and (3). According to the contextualism developed here, although epistemic standards vary with the context, the truth-value of particular knowledge-attributions does not. Contexts here are understood as being constituted by two elements: an epistemic practice (a rule-governed social practice such as a scientific discipline, the law, a craft etc., in which knowledge-claims are evaluated according to specific standards) and the "facts of the matter" (i.e. those facts which, together with the epistemic standards in question, determine which error-possibilities are relevant and thus have to be eliminated for a knowledge-claim to be true). If there are several epistemic practices, and thus several contexts, in which a knowledge-claim can be evaluated, it is the "strictest" practice that counts. In this way, the counterintuitive consequence of other versions of contextualism that the same knowledge-claim can be true in one context, but false in another, can be avoided. At the same time, scepticism can be resisted since even in the "strictest" epistemic practices, error-possibilities become relevant only when backed by positive evidence that they might in fact obtain. [ABSTRACT FROM AUTHOR]