 The principal goal of this entry is to present Frege’s Theorem in the most logically perspicuous manner, without using Frege’s own notation. Of course, Frege’s own notation is fascinating and interesting in its own right, and one must come to grips with that notation when studying Frege’s original work. But one need not understand Frege’s notation to understand Frege’s Theorem, and so we will, for the most part, put aside Frege’s own notation and the many interpretative issues that arise in connection with it. We strive to present Frege’s Theorem by representing the ideas and claims involved in the proof in clear and well-established modern logical notation. With a clear understanding of what Frege accomplished, one will be better prepared to understand Frege’s own notation and derivations, as one reads Frege’s original work (whether in German or in translation). Moreover, our efforts below should prepare the reader to understand a number of scholarly books and articles in the secondary literature on Frege’s work, e.g., Wright 1983, Boolos 1990, and Heck 1993, 2011, and 2012.