Friction Stir Welding of Ti-6Al-4V was performed on 5mm thickness sheet.  A wide range of processing conditions, such as spindle speed and feed rate, were tested and an experimentally determined process window was established for this given material, thickness and tooling configuration.  Temperature measurements were also taken in order to determine a relationship between process parameters and workpiece temperatures.  Peak temperatures measured in the weld nugget approached 1000 C, and vary depending on the weld parameters used.  The process window and temperature measurements were also compared to process models.  It was found that simple models can be used to predict trends such as peak temperatures and acceptable welding conditions.