The term “defect” is usually associated with something going wrong during materials synthesis or fabrication. Yet surface defects tend to be the most reactive sites in interfacial chemical reactions and the physical properties of extended solids (magnetism, electronic transport) can be usefully controlled by the nature of defects and disorder within the bulk crystal. The challenge is understanding how defects impact these phenomena and ultimately controlling the type and density of defect. In this talk, I will discuss my group’s efforts to map topological defects and subtle lattice deformations in moiré superlattices of 2D materials and exploit these to tailor interfacial electrochemical reactions. I will also discuss our efforts to understand the effects of intercalant superlattice disorder on magnetism in intercalation compounds of transition metal dichalcogenides.