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Metal halide perovskites (MHPs) are frontrunners among solution-processable materials for lightweight, large-area and flexible optoelectronics. These materials, with the general chemical formula AMX 3 , are structurally complex, undergoing multiple polymorph transitions as a function of temperature and pressure. In this review, we provide a detailed overview of polymorphism in three-dimensional MHPs as a function of composition, with A = Cs + , MA + , or FA + , M = Pb 2+ or Sn 2+ , and X = Cl − , Br − , or I − . In general, perovskites adopt a highly symmetric cubic structure at elevated temperatures. With decreasing temperatures, the corner-sharing MX 6 octahedra tilt with respect to one another, resulting in multiple polymorph transitions to lower-symmetry tetragonal and orthorhombic structures. The temperatures at which these phase transitions occur can be tuned via different strategies, including crystal size reduction, confinement in scaffolds and (de-)pressurization. As discussed in the final section of this review, these solid-state phase transformations can significantly affect optoelectronic properties. Understanding factors governing these transitions is thus critical to the development of high-performance, stable devices.