Several mineralogical components in Cretaceous/Tertiary (K/T) boundary clays have been well characterized. Shocked quartz with multiple sets of planar deformation features, stishovite, magnesioferrite crystals, and shocked composite silicate grains are all found in K/T boundary clays and claystones and are specific mineralogical impact signatures not assignable to other scenarios such as volcanism. Hollow spherules, altered lithic clasts, the distribution of Ir in the boundary clay unit, and the maximum size distribution of shocked quartz grains worldwide also do not conform to endogenic hypotheses of origin. The microstratigraphy and textural components are fairly well known, especially for the nonmarine claystones in the Western Interior of North America. All of these components can be shown to fit an impact scenario and therefore are best defined as impact signatures. Other proposed hypotheses fail to account for all of the known mineralogical and textural data, or they ignore the diagnostic impact nature of some of these signatures. Until a hypothesis can be proposed that accounts for all of these components and integrates them with the geochemical data as completely as the impact scenario does, these components should be interpreted and applied as impact signatures.