Greco argues that trust is essential to testimonial knowledge. He offers two arguments, both of which depend on the premise that at least some testimonial knowledge is transmitted knowledge. Knowledge transmission, however, essentially involves joint agency—and this in turn essentially involves interpersonal trust. Along the way Greco casts new light on social-epistemological categories, distinguishing two forms of reductionism about testimonial knowledge (source reductionism and transmission reductionism) and situating so-called “trust theories” of testimonial justification, such as the assurance view, within this framework. His chapter yields a new, finer-grained way to think about the epistemic value of trust, and through this, about epistemic value in general.