Rheological behavior of plastic melt is rather complex as it depends on various interactions. In single phase systems, it depends mainly on macromolecular structures of the polymer. In multi-phase systems, it depends on macromolecular properties of polymers, filler properties, interactions between polymers, interaction between fillers and the polymer(s) as well as interaction between the fillers itself. Inline monitoring of rheological characteristics for continuous plastic processes are realized mostly based on relative values – e.g. to monitor pressure values – or with inline viscosity measurement systems. Such viscosity measurement systems are meanwhile somehow established monitor devices for complex or expansive continuous plastic processes in the extrusion or compounding industry [e.g. Dynisco/Göttfert]. Due to the measuring principle of those inline viscosity measuring systems only shear viscosities are measured. Simplified systems - with a fixed throughput rate and capillary geometry – are also called inline MFI/MFR sensors, whereas some others can measure entire viscosity curves over a certain shear rate range. Further elongational viscosity characterizations of polymers or polymer systems are mainly conducted offline by using equipment like the Rheotens system [Göttfert et al.]. Beside time delay between production and measuring, also measurements in a molten state or at certain temperatures are somehow difficult. Within the scope of this contribution an online elongational viscosity measuring instrument and its advantages will be presented. Keywords: Online Rheometer, Elongational Viscosity, Smart Sensor, Hyperbolical Slit Die, .