Injection molding is a standard manufacturing process for plastic parts. The most important process step, mold filling, involves unsteady nonisothermal flow of a non-Newtonian molten plastic. Mold-filling flow largely determines molecular orientation within the final part and thereby influences final part properties. This article describes techniques for successfully applying particle image velocimetry (PIV) to molten plastic flow during injection molding. The primary experimental challenges include the following: engineering optical access to the molten plastic flow at elevated temperatures (230-245 degrees C) and pressures (similar to 20 MPa), finding particles that survive the thermal-mechanical environment that melts the plastic, and developing experimental and data-reduction techniques that allow multiple imperfect planar PIV measurements to be combined. Here, a custom optical-access mold allowed mold center-plane PIV to be performed in molten polystyrene. Simple statistical assessments of the velocimetry data and scaled residuals of the continuity equation suggest that the PIV can be conducted in molten plastics with an uncertainty of +/- 2%. POLYM. ENG. SCI., 51:730-745, 2011. (C) 2011 Society of Plastics Engineers