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Host molecules have been designed and synthesized to selectively complex and lipophilize guest molecules. Examples of the use of the following binding interactions are given: hydrogen bonding, ion pairing, cation to n-electrons, carbonyl to n-electrons and pi-pi bonding. Multiheteromacrocycles have been prepared whose association constants with tert-butylammonium salts in chloroform range from <50 to 106 M . Host molecules with built-in counterions have been prepared that selectively complex and lipophilize metal and alkylammonium cations. Locations of complementary binding sites and noncomplementary steric barriers provide for selective binding by host molecules of candidate guest molecules. Locations of appropriate chiral barriers and multiple complexing sites in guest compounds have led to the complete optical resolution of host compounds by optically active amino acids, and of amino acid esters by optically active host compounds. Ratios of association constants for diastereomeric complexes in excess of ten have been obtained. A molecular basis for designing an amino acid resolving machine has been developed.Central to nature's enzyme, transport and regulatory systems are highly structured molecular complexes. Large host molecules bind smaller guest molecules, and the chemical and physical properties of each are vastly altered. Nature's complexes are characterized by a high degree of structure, very high rates of formation and decomposition, and mutual structural recognition of host and guest. In enzymic catalysis, the rate-limiting transition state energies are lowered by complexation and orientation. In transport mechanisms, selection and lipophilization of polar entities are frequently accomplished by complexation. In regulatory systems, competitive complexation between inhibitor and substrate for sites of host molecules control *