Testimony is the mainstay of human communication and essential for the spread of knowledge. But testimony may also spread error. Under what conditions does it yield knowledge in the person addressed? Must the recipient trust the attester? And does the attester have to know what is affirmed? A related question is what is required for the recipient to be justified in believing testimony. Is testimony-based justification acquired in the same way as testimony-based knowledge? This paper addresses these and other questions. It offers a theory of the role of testimony in producing knowledge and justification, a sketch of a conception of knowledge that supports this theory, a brief account of how trust of others can be squared with critical habits of mind, and an outline of some important standards for intellectual responsibility in giving and receiving testimony.