Mineralogical and clay mineralogical investigations under certain circumstances allow conclusions regarding the rate and kind of pedogenetic weathering, especially clay mineral formation. Together with soil physical and chemical and mainly micromorphological investigations, paleoclimatic conclusions can be drawn. In Central European Holocene and Pleistocene loess soils as well as in relict loess soils in the central and northern part of the Great Plains of the U.S.A., smectite is the dominant pedogenetically formed mineral in the fine clay fraction (< 0.2 μm); illites are dominant in the coarse clay. In contrast in Holocene and Pleistocene loess soils of Central Asia (Kashmir Valley, loess plateau of Central China) mainly illites are formed pedogenetically and predominate in the coarse and fine clay fractions. Even in intensively weathered and rubefied Earthy Braunlehms (Rhodudalfs) of mid and old Pleistocene age, no, or only very small amounts of kaolinite were formed pedogenetically.