Simple chemical catalysts have been designed to achieve some desirable features of enzymes. These novel catalysts are not proteins, but they may incorporate the typical enzyme catalytic groups and they achieve selectivity in their reactions by use of geometric control, as do enzymes. Catalysts that carry out geometrically controlled chlorinations of aromatic rings and steroids have been constructed. Other catalysts achieve the selective synthesis of amino acids, and still others imitate ribonuclease in detailed mechanism and hydrolyze RNA. Optimization of geometries has led to a rate acceleration of over 10(8) in one instance.