This paper surveys the prominence hierarchy, a term which we use to refer to a ranking of person and other categories of reference for a variety of grammatical purposes. This kind of hierarchy has been referred to in the literature by various names, for example person hierarchy, empathy hierarchy, animacy hierarchy, indexability hierarchy, and so on. We exemplify the set of morphosyntactic phenomena accounted for by such hierarchies, and consider assumptions about how hierarchies are structured and how they work. We also discuss suggested motivations for the existence of the hierarchy, consider whether a universal statement of the prominence hierarchy can be made, and examine whether prominence hierarchies can be considered to be a typological universal. Finally, we discuss two examples of formal approaches to data described by prominence hierarchies.