This chapter explores the evolution of the symbolic boundary between the categories ‘professional’ and ‘non-professional’ translation and interpreting in translation and interpreting studies (TIS). It discusses categorisation and boundary work as fundamental principles in the development of human thought and perception, and the role they play in the development of research fields. In science and research, boundary work can narrow or expand our thematic perspectives, help delineate the differences between various concepts, prevent research into some topics while promoting others, and determine boundaries of legitimacy. In TIS, boundary work relating to the epistemological categories of professional and non-professional translation and interpreting has made an essential contribution to the field’s dynamics. This chapter illustrates how boundary work has led to the expansion and monopolisation of the field. Drawing on examples from historical contexts as well as recent research on non-professional practices, it also demonstrates how these practices can challenge the seemingly robust and impermeable demarcation line between the two categories.