In the friction stir welding (FSW) process, heat and mechanical work are coupled to produce a solid state weld. The process variables are pin tool rotation speed, translational weld speed, and downward plunge force. The strain-temperature history of a metal element at each point on the cross-section of the weld is determined by the process variables plus the individual flow path taken by the particular filament of metal flowing around the tool and ending on that point. The strain-temperature history determines the properties of a metal filament on the weld cross-section. To control the mechanical properties, the strain-temperature history must be carefully controlled. Indirect estimates of the flow paths and the strain-temperature histories of filaments comprising friction stir welds can be made from a model, if the model provides sufficient information. This paper describes experimental marker studies designed to trace the metal flow streamlines as influenced by variations in the process parameters.