Noncovalent associations between arenes and heteroarenes have important structural and functional consequences for organic synthesis, biological structure and binding, synthetic molecular recognition, supramolecular chemistry, π-conjugated materials, and crystal engineering. This Chapter, drawing on the most recent theoretical and experimental data, discusses the fundamental “nature” of (hetero)arene association and how the geometry and energy of the interaction vary in response to substituents, size, curvature, and solvent.