This introductory chapter to the special issue "Referential hierarchies and alignment" of Linguistics (54:3) gives an overview of the history and the different definitions of the notion "referential hierarchies" (a.k.a "animacy hierarchies" "hierarchies of inherent lexical content", "nominal hierarchies" etc.). It is furthermore gives a definition of what we understand by the term "alignment" and shows how referential hierarchies can influence alignment patterns. Finally, the articles represented in the volume are briefly summarized.