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Part-through crack behavior has been evaluated in one quarter, one half and one-inch thicknesses of mill-annealed Ti-6Al-4V plate. Constant-amplitude fatigue-crack propagation, overload-crack retardation, near-fracture crack extension, and critical fracture or residual strength were the four aspects of crack behavior considered. The near-fracture crack extension studies demonstrate that over the finite range of thickness considered, the region of critical crack instability occurs at the extremes of the crack length dimension, rather than at the depth as postulated by classical elastic concepts. This supports the representation of fracture data by a semiempirical approach outlined herein. In general, it appeared that toughness can be characterized by simple formulations of the stress-intensity factor, excluding the back surface correction. However, the back surface correction was a necessary transformation to achieve a smooth transition of fatigue-crack-propagation behavior during breakthrough of the back surface. (Modified author abstract)