A traditional start to this chapter might emphasize that anxiety disorders are common: Lifetime prevalence in adults in the United States for any anxiety disorder is estimated to exceed 20%, causing widespread suffering and impairment (Kessler, Petukhova, Sampson, Zaslavsky, & Wittchen, 2012; excluding disorders no longer considered anxiety disorders). Alternatively, we might take the well-worn tack that anxiety disorders are a significant public health burden that leads to vast economic expenditures in the United States and worldwide (Baxter, Vos, Scott, Ferrari, & Whiteford, 2014;Kessler & Greenberg, 2002). Yet another alternative would be to cite continued controversies in regard to the anxiety disorders; for example, whether it is really the case that some treatments for them are better than others (Siev & Chambless, 2007;Wampold, 2006). Indeed, once one accepts that there is a set of anxiety disorders, it is not difficult to find evidence that these disorders are associated with considerable distress, impairment, economic cost, and increased mortality as well as controversy regarding what should be done about their negative effects.However, starting this chapter traditionally would sidestep an essential issue: What, precisely, are anxiety disorders? A related issue involves how we understand psychopathology generally: How do we describe and define an anxiety disorder? We believe it is essential to answer this (admittedly somewhat abstract) question to frame the remainder of the chapter. Furthermore, what, if anything, determines that the anxiety disorders, as currently defined, should be grouped together? That is, what are the anxiety disorders, as a group?