 In the following we focus mostly on contemporary formulations of mereology as they grew out of these recent theories—Leśniewski's and Leonard and Goodman's. Indeed, although such theories come in different logical guises, they are sufficiently similar to be recognized as a common basis for most subsequent developments. To properly assess the relative strengths and weaknesses, however, it will be convenient to proceed in steps. First we consider some core mereological notions and principles. Then we proceed to an examination of the stronger theories that can be erected on that basis.