The chapter explores the metaphysical concept of structure in composite entities. It begins with a discussion on how entities, both natural and artificial, have parts organized in specific ways, introducing key aspects of structure: order, repetition, and stratification. The chapter delves into how these aspects interact with the roles parts play within an entity’s structure, linking this to their functions. The impact of structure on the identity and existence conditions of composite entities is also discussed. In particular, mereological extensionality, universalism, and nihilism are discussed from the viewpoint of structure. The chapter also examines the hylomorphist Aristotelian framework of form and matter, proposing that form can be understood as structure or function and discussing its role in the persistence of substances. Further, some kinds of plausibly structured abstract entities, such as propositions and facts, are discussed, based on their roles in semantics and truth-making. Structured propositions challenge extensionality due to their differing arrangements of identical parts.