We have developed a measurement methodology for measuring resin temperature and temperature profiles during processing using fluorescence spectroscopy. The technique consists of mixing a temperature sensitive fluorescent dye with a polymer resin at very low concentration, on the order of 10−6 molar concentration, and monitoring the temperature dependence of the fluorescence spectra. Two classes of fluorescent dyes are used: (a) excimer producing dyes such as bis-(pyrene) propane, and (b) band broadening dyes such as perylene and benzoxazolyl stilbene. Because the fluorescence measurement reflects the temperature in the neighborhood of the dye molecule, true resin temperature is observed and effects due the large thermal mass of the processing machine are minimized. Real-time monitoring was achieved using optical fiber sensors which were inserted into the process line at existing instrumentation ports. We monitored both injection molding and extrusion observing effects due to shear heating, crystallization and thermal diffusion. Temperature profiles in a flowing resin were measured using a sensor with confocal focusing optics. With this sensor, excitation light is focused to a point in the resin and the temperature at that point is deduced from the resultant fluorescence. In most cases, the fluorescence data must be corrected for effects due to pressure in order to yield an accurate temperature measurement.