Macrocycles and metallomacrocycles have been used in the design of high-affinity, selective receptors for a variety of substrates. Small molecule guest ligands can be bound to the metallomacrocyclic host through sigma-donor and pi-acceptor interactions. Furthermore, the macrocycle itself can adopt cleft-like conformations suitable for 3D encapsulation of substrates. Additional receptor groups can be readily appended to the macrocyclic complexes, in order to accomplish more complex multi-point guest recognition. The flexibility of the metallomacrocyclic receptors can be varied, so that either highly selective rigid hosts, or more versatile, flexible hosts can be obtained, as needed. Relatively flexible macrocycles with appended receptor sites can display pronounced shape selectivity in the molecular recognition of cationic or anionic guests. Combining metal ion reactivity with the recognition properties of metallomacrocycles appears to be very promising for the development of new switchable receptors, as well as new, selective reagents and catalysts. The field of molecular recognition using metallomacrocycles started out with small molecule binding within well-defined cavities. Later, the versatility of macrocyclic scaffolds provided numerous possibilities for cavity expansion. Advances in the design and preparation of polyfunctional macrocyclic platforms, which often contained deep cavities and/or several appended functional groups, resulted in an impressive array of supramolecular hosts for medium-sized guests. Examples included successful recognition of di- and polyamines, di- and polycarboxylates, and important biomolecules (such as neurotransmitters). Simultaneous binding of a small molecule (O2) and medium-sized organic molecules within the same polymacrocyclic cavity was also accomplished, opening the way toward low-molecular-weight models of enzyme active sites. Due to the ability of the metal centers to act as spectrophotometric, fluorescent, or redox reporters of binding events, these selective molecular hosts can be applied in sensor design. Molecular recognition using functionalized metallomacrocyclic hosts has also been employed in the design of molecular machines. Finally, macrocyclic compounds were recently shown to be effective in recognizing large guests, such as polyoxoanions, fullerenes, and carbon nanotubes. Hosts composed of several macrocyclic rings are often required for binding of these nanoscale objects. Alternatively, long tendrils extending from a monomacrocyclic scaffold may, through electrostatic, hydrogen bonding or π-π interactions, effectively "chelate" neutral or anionic guests. Just as the classic principles of coordination chemistry have recently been extended to the recognition of small anions (26), chemists are gradually developing a system and language for recognizing these even larger species.