This chapter argues (a) that macromolecules are fundamentally relational entities and (b) that only a process ontology can account for them. The basis for the argument is the ecological model proposed in 1981 by Charles Birch and John B. Cobb, which states that all entities, from atoms to populations, are ecosystems and hence fundamentally relational in character. The chapter first discusses how Birch and Cobb use the concept of internal relations to argue that ecosystems are processual in nature. It then shows that their account fails when it comes to macromolecules, as it does not offer an understanding of macromolecular behaviour in terms of internal relations. Following this, two case studies—symbiosis and enzyme function—are used for developing a fully relational account of molecular behaviour. This enables the expansion of Birch and Cobb’s ecological model into a process framework that also applies to macromolecules.