Galen employs several different taxonomies of body parts, dividing the body up in various ways. This chapter looks at some of the more prominent ones, especially those that either define parthood or shed light on Galen’s theorization of parthood in other ways. The central question guiding this discussion arises from claims about (in)expendability of various parts: why is it the case that, according to him, the loss of a bone leads to a complete loss of activity the bone supports (voluntary motion in a limb), but the loss of the stomach does not lead to the loss of the activity of nutrition. One of the key preoccupations emerging from various ways in which Galen differentiates bodily parts is the proper activity of parts, which shape his understanding of the role of parts and their significance relative to each other. The final sections of the chapter sketch out the difference between normative and functional understandings of a parthood.