Our knowledge of the world comes from various sources. But it is sometimes said that testimony, unlike other sources, transmits knowledge from one person to another. In this book, Stephen Wright investigates what the transmission of knowledge involves and the role that it should play in our theorising about testimony as a source of knowledge. He argues that the transmission of knowledge should be understood in terms of the more fundamental concept of the transmission of epistemic grounds, and that the claim that testimony transmits knowledge is not only defensible in its own right, but indispensable to an adequate theory of testimony. This makes testimony unlike other epistemic sources.