“Unity” has become a much-maligned word in history and philosophy of science circles, the subject of criticism that is both normative and descriptive. The concepts of “unity of science” and “unity of method” and even the notion of a “unified theory” have been criticized for being either politically undesirable (Dupré 1996) or metaphysically undesirable (Galison and Stump 1996), or else simply nonexistent – the products of a misrepresentation of scientific practice. Critics of unity claim that when we look at scientific practice we see overwhelming evidence for disunity, rather than the coherent structure we have been led to believe characterizes science. Although some of these arguments are extremely persuasive, the desire to banish unity altogether has resulted, I believe, in a distortion of the facts and a misunderstanding of how unity actually functions in science. It is simply a mistake to deny that science has produced unified theories. So where does the evidence for disunity come from? In order to answer this question, we need to look to theory structure as a way of clarifying the nature of that unity. The task then, as I see it, is not so much one of defending a strong version of unity at all costs, but rather of providing an analysis of how it is achieved and how it functions. To that end I have chosen to focus on theory unification as the basis for my discussion of unity. Not only do unified theories provide the foundation for a more general notion of scientific unity, but also there has been a great deal of attention paid to theory unification in the philosophical literature (e.g., Friedman 1983; Glymour 1980; Kitcher 1989).