A survey of the use of quaternary ammonium compounds as reagents and as catalysts in homogeneous and two-phase systems is given. The use of solvents and the influence of the structure of the onium compounds on their reactivity is mentioned. A comparison with the corresponding uncatalysed systems is briefly made. 1. Substitution Reactions 2. Elimination Reactions 3. Addition Reactions 4. Generation of Carbenes 5. Deuterium Exchange 6. Oxidation of Olefins with Potassium Permanganate 7. Sodium Borohydride Reduction 8. Influence of the Structure of the Onium Compounds 9. The Use of Ion-Exchangers as Catalysts 10. The Use of Solvents 11. Conclusion