Corroborative evidence may be understood as having two epistemic effects: a primary effect by which it offers direct evidence for some claim, and a secondary effect by which it bolsters the appraised probative, or evidential, value of some other piece of evidence for that claim. This paper argues that the bolstering effect of corroborative evidence is epistemically legitimate because corroboration provides a reason to count the belief based on the initial evidence as sensitive to, and safe from, defeat in a (...) way that it was not previously recognized to be. Discovering that our initial evidence tracks the truth in a way we previously did not recognize provides a reason to positively reappraise the probative value of that evidence. The final section of the paper relates the proposed sensitivity- and safety-based account of corroboration to an explanation-based account. (shrink)