  Crystals are a state of matter characterised by periodic order. Yet
crystalline materials can harbour disorder in many guises, such as
non-repeating variations in composition, atom displacements, bonding
arrangements, molecular orientations, conformations, charge states, orbital
occupancies, or magnetic structure. Disorder can sometimes be random, but more
usually it is correlated. Frontier research into disordered crystals now seeks
to control and exploit the unusual patterns that persist within these
correlated disordered states in order to access functional responses
inaccessible to conventional crystals. In this review we survey the core design
principles at the disposal of materials chemists that allow targeted control
over correlated disorder. We show how these principles---often informed by
long-studied statistical mechanical models---can be applied across an
unexpectedly broad range of materials, including organics, supramolecular
assemblies, oxide ceramics, and metal--organic frameworks. We conclude with a
forward-looking discussion of the exciting link to function in responsive
media, thermoelectrics, topological phases, and information storage.
