Thin-section/scanning electron microscope studies of thousands of sandstones representing many ages. compositions, and depositional environments indicate authigenic clays are far more common than previously recognized. Authigenic clays occur as pore linings, pore fillings, pseudomorphous replacements, and fracture fillings. Sandstones also contain allogenic clays. Allogenic clays originate as terrigenous material (dispersed matrix, sand-sized floccules, sand- to cobble-sized mud or shale clasts) or are introduced subsequent to deposition as a result of bioturbation or infiltration. An authigenic origin can be established on the basis of clay composition, structure, morphology and distribution, and sandstone textural properties. However, no individual criterion is an infallible indicator of authigenic origin. The most reliable criteria are a) delicacy of clay morphology, which precludes sedimentary transport, b) occurrence of the clay as pore linings absent only at grain contacts, and c) composition radically different than associated allogenic clays. Distinctions between authigenic and allogenic clays become difficult if the latter are recrystallized or if either is extensively deformed by burial or tectonism. Studies of authigenic clays utilizing a combination of thin-section, scanning electron microscope with nondispersive elemental analyzer, and x-ray diffraction show that each of the major clay groups exhibit a limited number of distinctive morphologies. Smectite occurs as highly wrinkled or honeycomb-like pore linings, the individual flakes of which are not resolvable. Illite forms pore-lining overlapping flakes whose edges tend to curl away from the grain surface and from which highly elongate lath-like projections may extend. Mixed-layer smectite/illite morphologies resemble those of both smectite and illite. Chlorite occurs primarily as pore-lining pseudohexagonal flakes with a cardhouse, honeycomb, or rosette arrangement. Kaolinite and dickite most commonly form pore-filling books of stacked pseudohexagonal flakes. Occasionally, they form thin pore-lining sheets of overlapping pseudohexagonal flakes. Authigenic clays are a major control on reservoir quality. Permeability and water saturation are particularly sensitive to the relative abundance of clays. Interpretations of depositional environment and provenanco based on sandstone composition and texture may be inaccurate if the presence of authigenic clay is overlooked.