In 1967, Charles J. Pedersen reported the discovery of a new class of macrocyclic compounds, the crown ethers, 1 and their ability to bind alkali metal ions within their cavities that would later be described as if the "ion had fallen into the hole in the center of the molecule." 2 Jean-Marie Lehn's subsequent work on bicyclic analogues of the crown ethers -named cryptandsdemonstrated the power of extending these host systems into three dimensions (Fig. ). This work would see Pedersen and Lehn, 5 alongside Donald J. win the 1987 Nobel Prize in Chemistry "for their development and use of molecules with structure-specific interactions of high selectivity." The concept of host-guest chemistry had been born. Since this seminal work (and inspired by increased understanding of natural architectures like enzymes), chemists have been fascinated by the idea and challenges of preparing three-dimensional artificial host systems, often termed capsules, cages or containers. These include covalent systems (of which cryptands are arguably the progenitors), which can incorporate dynamic covalent bonds (such as imines), as well as those assembled using non-covalent (supramolecular) interactions.