The type of undulations we show in figure 1 are not the only ones that one encounters in practice. Depending on the geometry and type of polymer, various other types of distortion can occur. The short wavelength deformations of the interface often refered to as "sharkskin" instability appear to originate at the outlet: the extrudate quasi-periodically sticks to the outlet, widens, snaps loose and narrows. In the "spurt-flow" regime, the extrudate shows intermittent bands of smooth and irregular surfaces; there is good evidence that this has to do with a stick-slip phenomenon at the wall of the die. In spite of the multitude of possibilities, there is every reason to believe that when these instabilities are absent, as in the experiments of figure 1, polymer flow exhibits some generic bulk flow instability: According to the engineering literature, a qualitative change in the flow behavior appears to occur at a more or less constant ratio of the normal stress difference of the melt over the shear stress for almost any polymer.