This book investigates whether knowledge is closed under known entailment. Traditionally it has been assumed that if a person knows some proposition p and also knows that this proposition entails another proposition q, then by inferring q from p that person would gain knowledge of q. This so-called 'Principle of Deductive Closure' is of intrinsic interest because, if true, it expresses an important structural characteristic of knowledge. Challenges to this principle of deductive closure have been formulated by Fred Dretske and Robert Nozick, among others. Most replies to these challenges, as well as the challenges themselves, make explicit or implicit use of the idea that our knowledge claims are not invariant, but relative to a context. Therefore, a substantial part of the book is devoted to an analysis of contextualism and a criticism of the current contextualistic accounts. Once developed, the account is then used to answer the challenge to the principle of deductive closure. Epistemic contextualism results in a limited closure principle.