 When thinking about the question of reducibility in chemistry, it is useful to break this question into two parts: The first, and more familiar one to philosophers, concerns the relationship between elements, atoms, molecules, and the fundamental particles of physics. We might ask, “Are atomic and molecular species reducible to systems of fundamental particles interacting according to quantum mechanics?” A second, less familiar question concerns the relationship between the macroscopic and microscopic descriptions of chemical substances. “Are chemical substances reducible to molecular species?” Here, the main question is whether all chemical properties that have been defined macroscopically can be redefined in terms of the properties of atoms, molecules, and their interactions.