The distribution and origin of clay minerals in Carboniferous sandstone reservoir rocks, onshore and offshore UK, is reviewed using both published and unpublished sources. The clay mineralogy for many of the Carboniferous reservoir sands tends to be similar, with the detrital clays predominantly illitic whereas the diagenetic clay assemblages are dominated by kaolin with usually lesser amounts of illite. The main exception to this pattern is found in the Dinantian sandstones of the Clair Basin where significant amounts of smectite are present. Three stages of diagenetic kaolin formation are widely recognized. Firstly eogenetic and/or telogenetic kaolinite; secondly mesogenetic kaolin; and thirdly the partial or complete transformation of kaolinite to dickite during deep burial. In addition to the formation of diagenetic clay phases, the sandstone reservoirs also display a complex diagenetic history involving cementation and dissolution processes. These have affected the reservoir properties of the sandstones but the depositional facies architecture still exerts a major recognizable influence on reservoir porosity-permeability characteristics. The abundance of kaolin cements shows no clear correlation with variations in porosity and permeability for Carboniferous reservoirs. Pore-filling smectite affects reservoir porosity and permeability in the Dinantian of the Clair Field, and could be a potential source of serious reservoir damage arising from swelling.