Fault rocks can be studied by charting how undeformed rocks near a fault transform into mylonitic or cataclastic tectonites, or by examining rock masses at different points along a fault to determine how changes in temperature, pressure, etc. affected the fault’s history. Both approaches have merit in thrust belts because thrust faults form under a range of conditions and may evolve along several different paths. Using the first approach, we distinguish two fault zone types analogous to Means’ (1984) two types of shear zones: Type I fault zones grow in thickness as movement on the fault increases; Type II fault zones initiate as zones of localized deformation, and deformation becomes further localized as displacement increases. Both Type I and Type II fault zones occur in the Appalachian fold-and-thrust belt. The second approach shows that fault rocks from the thrust zone beneath the southern Appalachian Blue Ridge and that beneath the Bay of Islands ophiolite evolved in similar ways, despite differences in rock types and local structural history. Three conclusions emerge from our survey of fault rocks from thrust faults: (1) rocks from both external and internal thrust zones may deform by fracturing or by plastic flow, and may alternate between those modes as local physical conditions change; (2) fault zones with large displacement nearly always weaken with continued displacement; (3) fluid phases are critically important to the softening processes, which accommodate large displacements in both external and internal thrust zones.