Ethos is a key concept in rhetorical criticism lending insight into a transcendent feature of human communication. To better understand how ethos can be used as a tool of criticism, this chapter starts by comparing ethos and personal credibility. Drawing on rhetoric’s Greco-Roman origins, the chapter then describe how rhetorical critics use ethos to understand techniques of rhetorical invention and how those techniques inform audience constitution—far beyond the rational decisions of the rhetor. The chapter then turns to Swedish climate activist Greta Thunberg as a case study revealing how her potent ethos was put into action not only by what she said, but by who she was. The chapters concludes with a discussion of the ethical considerations for the rhetorical critic keen on using ethos as a critical tool.