 Our contemporary understanding of chemical substances is elemental and atomic: All substances are composed of atoms of elements such as hydrogen and oxygen. These atoms are the building blocks of the microstructures of compounds and hence are the fundamental units of chemical analysis. However, the reality of chemical atoms was controversial until the beginning of the 20th century and the phrase “fundamental building blocks” has always required careful interpretation. So even today, the claim that all substances are composed of elements does not give us sufficient guidance about the ontological status of elements and how the elements are to be individuated.