Messier 33 (= NGC 598) in the constellation Triangulum has the largest angular size of any late-type spiral galaxy. Only three other extragalactic objects, the Magellanic Clouds and Messier 31 in Andromeda, exceed the apparent dimensions of M33. And only M31 is also visible from the northern hemisphere. The large size and favourable inclination of M33 make it an excellent object for study. Our own Galaxy, the Milky Way, has traditionally been compared to the Andromeda galaxy. However, several recent observational results indicate that the Milky Way may be later in morphological type than M31. Hence the best model for our Galaxy may turn out to be a judicious interpolation between models obtained for M31 and M33.