The working title of this Handbook presumed three neatly bounded territories: science, technology, and society. This chapter makes those territories and especially their borders into objects for sociological interpretation and seeks to recover their messiness, contentiousness, and practical significance in everyday life. Its focus is on the "boundary problem" in science and technology studies: Where does science leave off, and society — or technology — begin? Where is the border between science and non-science? Which claims or practices are scientific? Who is a scientist? What is science?