This Element is a systematic study of the question of whether classes are composed of further parts. Mereology is the theory of the relation of part to whole, and we will ask how that relation applies to classes. One reason the issue has received attention in the literature is the hope that a clear picture of the mereology of classes may provide further insights into the foundations of set theory. We will consider two main perspectives on the mereology of classes on which classes are indeed composed of further parts. They, however, disagree as to the identity of those parts. Each perspective admits more than one implementation, and one of the purposes of this work is to explain what is at stake with each choice.