Soil mineralogical information is of primary importance in the characterization and utilization of soils. Since the first report of clay mineralogical data for Canadian soils in 1937, mineralogical data for approximately 1500 samples from 500 pedons in Canada have been obtained and recorded. All these data were assembled along with data for related bedrock samples. Criteria for mineralogical classes were defined and the existing data were classified and compiled in digital files. A part of the data were the spatial coordinates allowing the site observations to be input and manipulated within a Geographical Information System (GIS). The mineralogical data were used in conjunction with the Soil Map of Canada (1:5 million) to produce a clay mineralogical map of surface soils (1:10 million) which is attached to this monograph. When many clay mineralogical data are available such as the case in southwestern Ontario, the data were used with regional Soil Landscape Maps (1:1 million) to produce a more detailed clay mineralogical map of the region. This map is now being used as a layer of information in the GIS and also in plotted form to study the relationship between clay mineralogy and pesticide use for areas of intensive crop production in this region. During the process of compiling the existing clay mineralogical data and manipulating them in a GIS, a number of gaps in the information were identified particularly for noncrystalline and smectitic soil components. New studies are currently in progress to fill in these information gaps.