Although bibliography, textual criticism, and book history are parts of a common enterprise, each employs distinctive approaches and makes unique contributions. The nature of their connection, meanwhile, can best be grasped by first recognizing that each has both a specific and an extended meaning. What they share fundamentally is their historical outlook, all of them being attempts to understand and reconstruct the past. These considerations lead to a definition of the most problematic term, book history, as the study of every aspect of physical books at every point in their history. Observing some simple principles can enhance scholarship in these three kinds of intellectual activity that form a continuum within a single field.