One of the facts of the world is that things are made up from, or constituted by, stuff. Things are indicated by count terms; stuffs are indicated by mass terms. It would therefore seem that in order to give an adequate characterization of the world one must show how things can be thus constituted; and (if one wants to use logical tools in describing language) one must show what are the differences between count and mass terms, and how they are related. One kind of change occurs when one and the same object is constituted by different stuffs at different times; another kind of change occurs when one and the same stuff constitutes different objects at different times. How is this to be explicated? This anthology analyzes these changes and contains articles representing all the major viewpoints on such issues as: What is the mass/count distinction? Is it philosophically important? What is the designatum of a mass term? What is the logical form of sentences containing mass terms? What is it to be 'constituted by' a stuff? How is change to be analyzed? And so on.