Temperature, pressure, time, stress, and flow geometry are the physical features that determine rheology. Even in simple flows, such as capillary extrusion, the temperature, pressure and stress history may be complex. Provided we understand what those factors are and how they influence the flow, one can often ignore the complexity by limiting analysis to a pragmatic consideration of apparent viscosity. The same simplification cannot be presumed where flows of complex geometry are considered; e.g., in a processing operation we are concerned with shaping a material and the “shape” of the flow is the most important physical factor.