During the last 15 to 20 years, our understanding of the geologic significance of the clay minerals has increased from nearly zero to the point where we can begin to make some practical application of our knowledge. Following is a brief list summarizing our general knowledge. 1) We know the clay mineral composition of most of the formations in the United States. 2) Pre-middle Mississippian clay suites are less complex than those in younger sediments. 3) The composition of clay suites may change drastically over an interval of a few feet or remain nearly constant throughout 5,000 ft. of section. 4) Both source and environmental information can be obtained. 5) Expanded clays are contracted by burial. 6) The clay suites in limestones are as variable as those in shales. 7) Clay minerals in sandstones are more apt to have been altered after deposition than the clays in other rocks. 8) Once the clay mineral distribution pattern is established, reasonable environmental identifications can commonly be made. 9) If the environmental significance of the clay suites can be established by other means, the clay data can be used independently to identify environments near the control area. 10) Clays can best be used for environmental determinations in young sediments where deposition is relatively slow and environments are materially different. 11) Clay can be used for approximate "time correlation" where bentonite beds or unconformities are present and where there is a change in source material. 12) Relatively minor clay differences can be correlated for 10-50 mi. In the Upper Mississippian and Lower Pennsylvanian of the Midcontinent, the clay suites reflect the source areas and tectonic history. Many formations in this interval contain distinctive clay suites which can be used for identification. Detailed correlation can be made by making detailed studies.