Testimony, like memory, is an extremely pervasive source of knowledge that has traditionally been neglected by epistemologists. Here, I use “testimony” broadly, to include all cases in which a person asserts something, and another person hears, reads, or otherwise witnesses the assertion. In this sense, my beliefs that China is in Asia, that the Earth orbits the sun, and that my friend’s birthday is on June 29, are all based on testimony. Testimony also plays a crucial role in science, where scientists’ testimony as to their observations is relied upon by other scientists who are constructing theories. Yet, little has been written about the epistemology of testimony.