The heat input to the tool has been investigated for friction stir welding (FSW) of aluminum alloy AL 7075-T7351 over a wide range of process operating parameters using a combined experimental/numerical approach. In a statistical Design of Experiments fashion, 54 experimental welds (bead-on-plate) were performed at 27 different parameter combinations. Measured outputs during each of the welds included forces in all three coordinate directions and internal temperature of the rotating tool at three locations near the tool/workpiece interface. The heat input to the tool was also identified for each weld using infrared imaging temperature measurement techniques and the portion of the total mechanical power entering the tool was calculated. These values were subsequently analyzed to identify the effect of process operating parameters. Two-dimensional, axisymmetric numerical heat conduction models of the tool were then produced and the approximate spatial distribution of the heat input to the tool along the tool/workpiece interface was identified. Experimental values for the heat input to the tool ranged from 155 W to 200 W, comprising 2.8% to 5.1% of the total mechanical power. Regression equations developed for the two values show that each is a function of the process operating parameters. Heat conduction models of the tool show that the approximate spatial distribution of the heat input to the tool along the tool/workpiece interface is one where the heat input is distributed non-uniformly along the interface, with 1% entering the tool at the pin, 20% entering at the base of the pin, and the remainder entering the flat portion of the shoulder. This distribution was valid for the majority of process operating parameter combinations tested. The maximum predicted temperature for the simulations occurred in the pin. This result was verified by the experimental tool temperature measurements. Insights gained into the FSW process from the combined experimental/numerical investigation were then discussed.