and can then be compared directly to experimental results, thus, allowing for a direct validation of computational approaches. The well-defined structure also makes a proper physico-chemical characterization of these materials possible and provides, thus, the basis for the high level of understanding of their structure and electronic structure. At the same time, crystal order is the reason for solid-state effects that are caused by the translational symmetry, such as magnetic order, indirect band gaps, and ballistic charge transport. As in other crystalline solids, defects play a critical role for certain properties, including catalytic activity, electronic and optical properties, thus, a proper characterization of defect types and their density is crucial. This point is particularly important, as the properties discussed below are a direct consequence of the crystallinity of the framework materials.