There is a dichotomy in the information on flow of anisometric particles. Most of the fundamental studies only consider dilute suspensions in Newtonian liquids, although some authors venture into a semiconcentrated (two-body collision) region and others into pseudoplastic liquids. These publications provide a solid base for understanding the behavior of the high-concentration systems of industrial importance, but without the desirable quantification. The description of these systems is experimental or, at best, qualitative, via simplified constitutive models. At high concentration of anisometric particles, one must consider: yield stress, plug flow, shear segregation, and a change of relaxation spectrum. There is no simple method to correlate the steady-state and dynamic test data. The magnitude of the stress overshoot in transient tests increases with concentration and deformation rate. While the normal stress increases with concentration, the die swell decreases. The yield stress in elongation is larger than that in shear, and the maximum strain at break initially increases with addition of filler, goes through a maximum, and falls to very low values at high loading. The orientation of anisometric particles can be accomplished in converging and diverging, i.e., extensional flow. In a simple shear field, the effect depends on the rate, concentration, and matrix viscosity—in general, shearing causes disorientation of aligned particles. All these effects influence melt processing. For extrusion, the plug flow narrows the range of processing variables, increases the solid-conveying zone, and may lead to flow instability. In injection molding, gating, pattern of orientation (modulated by solidification), and the transient effects depend on the specificity of the rheological behavior of the filled pseudoplastic liquids.