It is with great pleasure and satisfaction that we introduce this volume which comprises the papers accepted for the 4th International Conference on Hydrocyclones held in Southampton from 23rd to 25th September 1992. As the name implies, this is the fourth Conference in the series, with the previous ones held in Cambridge in 1980, Bath in 1984 and Oxford in 1987. The papers cover a wide span of activities, from fundamental research to advances in industrial practice and, as in the earlier volumes, make a significant contribution of lasting value to the technical literature on hydrocyclones. Hydrocyclones continue to widen their appeal to engineers; besides their traditional role in mineral processing they now attract a lot of attention in chemical engineering, the oil and gas industry, power generation, the food industry, textiles, metal working, waste water treatment, pharmaceuticals, biotechnology and other industries. The reason for this continuously increasing attention is, as David Parkinson (General Manager of Conoco (UK)) said recently, that". a hydrocyclone is an engineering dream, a machine with no moving parts." Yet as this Volume clearly shows, the hydrocyclone can do so many things and do them well, whether the application is in solid-liquid, liquid-liquid or liquid-gas separation.