Aside from introductory comments concerning the advantages of closure denial and the argumentative strategy of the book, this chapter primarily concerns the definition of closure. After considering some standard definitions, I introduce three crucial distinctions: knowledge closure versus warrant closure, closure versus transmission, and transmission versus penetration. I argue that shifting attention from knowledge closure to warrant closure is not only perspicuous but necessary if prominent closure-defending views – Crispin Wright’s, in particular – are to be accommodated. Warrant transmission occurs when S acquires a warrant for Q in virtue of her recognition that it follows from warranted P. Since closure doesn’t specify S’s source of warrant for Q, transmission implies closure, but closure doesn’t imply transmission. The distinction between closure and transmission underlies much of the subsequent discussion. I argue that transmission doesn’t require penetration, wherein S’s warrant for P itself suffices as a warrant for Q. This undermines one of Dretske’s arguments against closure; but it also undermines certain arguments in favor of closure. The end results are definitions of knowledge closure, warrant closure, and warrant transmission that are used throughout the rest of the book.