The diversity of MIM structures that can be accessed make them one of the most tuneable classes of host systems; the type, number and manner of interlocking of the components are all variable, as are their covalent structures, allowing the installation of myriad recognition and solubilising groups. Anion-binding MIMs, for example, have been prepared with recognition groups that interact via hydrogen bonds, halogen bonds, chalcogen bonds and can be charged or neutral. 227 Furthermore, so-called "impossible" MIMs have been reported that lack specific binding motifs. External functionalisation of MIMs has also allowed their incorporation into polymeric materials, resulting in soft matter with switchable properties dependent on the binding of a guest within the MIM cavity. Mechanical linking of the components means that covalent bonds must be broken to separate them, making MIMs highly robust. As hosts of small, often charged, species such as cations and anions, MIMs are incredibly versatile hosts that can be engineered with high precision. Although the potential for multiple co-conformations has been exploited in the design and synthesis of molecular switches and machines, 230 with more complex systems, and for binding of larger guest molecules, this flexibility may be detrimental to strong and selective recognition events. MIMs are usually readily characterised using standard techniques for covalent molecular systems: NMR and MS in solution, with NOESY often being useful for identifying co-conformations of the interlocked components, and SCXRD in the solid-state, all of which can also be used to probe host-guest interactions.