Among the armory of functional groups capable of binding anions,1-41, 2, 3, 4 the amides and the ureas are the most commonly employed anion binding sites in neutral organic anion receptor species (Fig. 1a, b). Oxyanion binding in proteins occurs through hydrogen bond donation from, most commonly, the peptide backbone NH groups and from arginine.5&65, 6 One challenge that has been increasingly well met by the supramolecular chemistry community over the last 15 years, has been to design anion receptors with increasing selectivity for particular anionic guests. Of course, the supramolecular chemist does not (yet) have the precise control over the positioning of functional groups in three-dimensional space offered by the tertiary structure of a protein. In spite of this, great progress has been made in the design of neutral anion receptors, with systems now displaying exceptionally high association constants with anions. One of the major design criteria used in developing these systems is to prevent self-association (Fig. 1c) that competes with anion complexation and, hence, lowers association constants. Fig. 1. (a) Amides and thioamides are excellent hydrogen bond donors to anions; (b) ureas and thioureas are complementary hydrogen bond donors for carboxylates; (c) amides can self-associate so lowering their affinity for anions; (d) Crabtree's isophthalamide 11-chloride complex; (e) Leob's upper-rim amide functionalized calix[4]arene-benzoate complex; (f) a schematic diagram of Gale and Loeb's receptor 18 forming a 1:2 receptor:nitrate complex; (g) crystal structure of the perrhenate complex of receptor 18; (h) a squaramide-acetate complex; (i) crystal structure of the sulfate sandwich formed by receptor 20 and the anion; (j)a phenolate anion bound to a tetralactam macrocycle—a precursor to rotaxane formation.