In this report, we have outlined a number of scientific challenges which represent building blocks for the comprehensive understanding of disperse flows encountered in a variety of technologies and in nature. In dilute particle-laden turbulent flows, we see a need for more realistic descriptions of particle-fluid and particle-particle interactions that would ultimately be suitable for incorporation in large eddy simulations. Experiments are needed to clarify the mechanisms by which particles modulate turbulence. Progress in computer simulation methods and kinetic-theory analyses are leading to new opportunities to obtain the equations of motion for more concentrated multiphase flows which are strongly influenced by or dominated by the disperse phase. However, instabilities inherent to the multiphase nature of these flows lead to the very complex behavior of industrial scale multiphase flows. We require a better understanding of these instabilities as well as coarse-grained models for the average multiphase flows at larger scales. A major issue in dense granular flows is to elucidate the manner in which particle-particle frictional interactions can lead either to mixing or segregation of different particle species. (C) 2003 Elsevier Ltd. All rights reserved.