This article presents a review of the development of research on the persuasive effects of evidence. Focusing on research dealing with the impact of evidence on persuasive communication, this review makes three related arguments. First, past claims about the inconsistency in evidence research are largely mistaken when testimonial assertion evidence is involved; second, the conditions under which evidence should produce maximal effects can be understood by reference to Petty and Cacioppo's Elaboration Likelihood Model; finally, persuasive effects are surprisingly consistent across evidence types when careful comparison of operational definitions, rather than conclusions, is undertaken.