Recent work has demonstrated that particles will migrate across streamlines in the viscous flow of concentrated suspensions due to gradients in concentration and gradients in the applied shear stress. Such migration processes are particularly important in the casting of composite materials, such as in the injection molding of filled polymers and in the formation of solid rocket propellant motors. New research has also demonstrated that bidisperse suspensions (suspensions of particles which have two distinct sizes) and polydisperse suspensions may undergo shear-segregation can lead to a substantial effect on the burn characteristics. Dr. Leighton's group is currently investigating this segregation phenomenon and developing a constitutive equation for the migration capable of predicting size segregation in complex flow geometries.