The use of various catalysts in the asymmetric addition of HCN (or an equivalent) to achiral aldehydes and ketones, giving optically active cyanohydrins is discussed. The catalysts include enzymes, chiral polymers, chiral titanium and boron complexes, and cyclic dipeptides. Special emphasis is given to the synthetic and mechanistic aspects of the reactions catalysed by the diketopiperazine cyclo-[(S)-His-(S)-Phe] (4). 1. Introduction 2. Use of Enzymes 3. Use of Polymers 4. Use of Organometallic Reagents 5. Use of Peptides 6. Mechanistic Aspects of the Reaction Catalysed by Cyclo-[(S)-His-(S)-Phe] 7. Miscellaneous 8. Summary