Jurisdiction refers to those tasks or responsibilities that are seen as central to and exclusively controlled by a profession. When library work is examined, what is the proper jurisdiction for professional, masters-level librarians? This study examines the definition of professional with respect to library workers by using data from a national survey of competencies for library support staff and by comparing American Library Association-approved competencies for beginning MLS librarians and certified support staff. According to this analysis, professional librarians are those who know context (history, theory), do research, educate patrons, and manage people and collections. They are not necessarily those who provide direct services.