Since the late 1930s, intellectual freedom has been a central theme in the professional ethics of librarians. From it has come powerful and inspiring rhetoric, but also confusion and controversy. This paper traces librarianship's notions of intellectual freedom to a widely analyzed concept in law and political science known as the marketplace of ideas, and finds that taking this broad theoretical view of intellectual freedom offers some useful insights into its strengths and weaknesses as an ethical cornerstone of the profession.