It is the strong interaction that is responsible for forming nuclei, the cores of atoms, by binding protons and neutrons together. The same force, albeit with a quite different mechanism, also creates protons and neutrons by forming bound states of quarks and gluons. The strong interaction is still the least understood part of the Standard Model—even though the underlying theory, quantum chromodynamics (QCD), has been known for several decades. The difficulty lies in performing calculations with this theory. In this chapter, we explain why this is the case, explore the surprising phenomena that arise from quantum chromodynamics, and discuss the experimental and theoretical methods we use to address the unresolved questions. For example, individual quarks cannot be extracted from a proton or neutron—they are always confined in bound states and can only occur in very specific combinations. In contrast, systems bound by electromagnetic forces, such as atoms, can be easily separated—an electron can be removed from an atom. Even a deuteron can be split into its constituent proton and neutron. The comparison of electromagnetism and the strong interaction offers valuable insights into these distinct behaviors.