friend or foe?*

MnBi₂Te₄ and related compounds were intensively investigated in the last three years as the first family of intrinsic magnetic topological insulators, where an ideal marriage of non-trivial electronic topology and magnetism makes them a promising materials platform for exotic quantum phenomena. Like other crystalline solids, however, MnBi₂Te₄ has defects in its crystal lattice that can affect both magnetic and topological properties. Identifying these defects and understanding their behavior are critical in fine tuning of the magnetic and topological properties of MnBi₂Te₄. In this presentation, I will talk about the type, concentration, and distribution of lattice defects with special emphasis on magnetic defects and their effects on the physical properties.  I will discuss how these magnetic defects can be employed to engineer the magnetic and topological properties of MnBi₂Te₄ and related compounds.  I will also present our recent progress on the control of lattice defects during materials synthesis.