This chapter introduces and discusses the main principles and controversies in mereology, the study of the relations between parts and wholes, focusing on the principles governing composition and parthood. It begins by distinguishing between composition as a relation among entities and as an action, introducing the principle of Uniqueness of Composition, which states that a plurality of components forms at most one composite entity. The challenges to this principle are discussed through metaphysical frameworks such as perdurantism and endurantism, which address identity over time and the relativization of properties. After that, two mereological theses that warrant forms of ontological economy are discussed. First, the principle of Extensionality of Proper Parts, which ensures ontological economy by requiring that composite entities with identical parts are on their turn identical; second, the more controversial thesis of Composition as Identity, which holds that composite entities are literally identical to their components. The chapter also addresses the problem of the many, which arises from the difficulty in defining precise boundaries for entities. Finally, it analyzes Mereological Universalism, according to which all pluralities of entities compose something, and nihilism, which denies the existence of composites altogether.