Originating from the challenges of using ordinary methods for determining the structure of salen-type nickel(II) complexes and the limitations of crystallography of unknown organic powders, which has not been reported for many years, this chapter mainly reviews crystal structure prediction, polymorphism, phase transitions, and the application of computational methods. It also states that new research is incorporating computational chemistry for analyzing powder XRD. In particular, we will explore the possibility of predicting polymorphic crystal structures, which are of academic and industrial interest and applying them to metal complexes and organic-inorganic hybrid materials. There are also high hopes for the recent development of electron diffraction.