The microstructural evolution occurring during friction stir welding of a near-a titanium alloy, Ti-5111, has been examined by backscattered electron imaging and electron backscatter diffraction. The unaffected baseplate (BP) microstructure consists of millimeter-scale prior b grains containing~100 lm large colonies of aligned a laths, related to each other by a strain-accommodating Burgers orientation relationship. The a laths are separated by fine, 100 to 150-nmthick, interlath b ribs. A heat-affected zone (HAZ) is observed~1.5 to 2.5 mm from the tool surface, characterized by a thickening of the b ribs and the formation of secondary a platelets within them closer to the tool. There is a narrow thermomechanically affected zone (TMAZ), comprised of outer and inner regions, observed~1.0 to 1.5 mm from the tool surface. Deformation is first observed in a~200-lm-wide outer TMAZ, where the microstructure is refined through an increase in fine secondary (a laths) a laths and the lattice orientations rotate to align the close-packed h11 " 20i directions with the shear direction (SD). Continued deformation closer to the tool produces periodic shear bands within a~300-lm-wide inner TMAZ, resulting in alternating regions of material that are deformed below and above the b transus. Material in the stir zone (SZ) within~1 mm of the tool surface consists of fine (~10 to 20-lm diameter) equiaxed prior b grains that are delineated by~500-nm-thick a and contain 150-500-nm thick a laths. The texture exhibits both D 1 ð " 1 " 12Þ½111 bcc and P 1 ð1 " 100Þ½11 " 20 hcp shear texture components, indicating that this material exceeded the b transus during welding.