Rheology goes hand in hand with flow simulation in process design and optimization. Rheological measurements of polymer viscosity, wall slip and melt elasticity accompanied by simulations are also very useful in problem identification and solution, as it will be explained with several examples encountered in industry.In injection molding of HDPE twist-off caps, for water and soft drink bottles, it was shown that the bottom cap ring could be filled only by resins having adequate levels of wall slip velocity. This was due to the requirement of very high shear rate flow through micron diameter connecting "bridges" between top and bottom parts. In injection molding, for production of traffic lights housings, premature stress cracking of PC was identified by viscosity measurement, without the need of flow simulation of cavity filling. In extrusion through a spiral die, for production of HDPE film, computer simulation identified the flow geometry that was responsible for defective film. In extrusion of PP, for production of large diameter pipe, computer simulation identified the screw design as the root cause of pipe failing compression testing. Measurements of first normal stress difference and flow simulation identified the reason for appearance of waviness on the surface of extruded and subsequently calendered PMMA sheet.