Disorder is especially common in solvate forms, which are commonly encountered in pharmaceutical chemistry; for example, it has been estimated that up to a third of all active pharmaceutical ingredients (APIs) can form hydrates. Solvates are frequently encountered when the host crystal lattice packing is relatively inefficient, resulting in channels or voids in the structure that can be filled by small molecules, reducing the free energy of the crystal compared to the non-solvated form. Moreover, the solvent is frequently dynamic, potentially providing entropic stabilisation. Hence, disorder may be intrinsic to the stability of the phase and not necessarily a marker of instability. Even if solvate forms are not the final API, they are frequently encountered during pharmaceutical production, and so understanding such forms is important for the characterisation of a drug substance and its production.