This revised publication replaces The Canadian System of Soil Classification (second edition) published in 1987. The changes incorporated in this current publication are based on the work of the Soil Classification WorkingGroup (SCWG) formerly of the Expert Committee on Soil Survey, and continued by the Land Resource Division of the former Centre for Land and Biological Resources Research. Current taxonomic activities are part of the Land Resources Program of the Eastern Cereal and Oilseed Research Centre, Ottawa, Ontario. Highlights of the third edition: A tenth order, Vertisolic, has been added; Subgroups intergrading to the Vertisolic Order have been added in the Chernozemic, Gleysolic, Luvisolic, and Solonetzic Orders; Major revisions have been carried out to the Cryosolic Order; A complete key to soil classification has been added in chapter 3. The main initiative for establishing the Vertisolic Order resulted from the Sixth International Soil Correlation Meeting on Characterization, Classification, and Utilization of Cold Aridisols and Vertisols organized by the United States Department of Agriculture. Another key initiative for establishing the Vertisolic Order in Canada was the decision by the United States Department of Agriculture to establish a new suborder Cryert in the Vertisol Order of U.S. Soil Taxonomy that allows clay soils with Vertic properties that occur in cold climates to be classified as Vertisols. In light of these events, a review of the classification of clay soils in Canada by the Soil Classification Working Group led to the recommendation to establish the Vertisolic Order. The main reasons for modifying the Cryosolic Order stemmed from the results of an International Correlation Meeting on Permafrost Affected Soils. In addition work by Charles Tarnocai, Ottawa, and Scott Smith, Whitehorse, who assisted the International Gelisol Working Group in establishing the Gelisol Order for the U.S. Soil Taxonomy led to the modification of the Cryosolic Order. The most significant changes include adding several new subgroups in both the Turbic Cryosol and Static Cryosol great groups and revising the description of all subgroups to make them as uniform as possible and to clearly identify those properties diagnostic of the particular subgroup. Another new and useful addition to the third edition is the introduction of a Key to Soil Classification, which provides keys to soil orders, soil great groups, and soil subgroups. Complete definitions of each order, great group, and subgroup are contained in the chapter on each order. All the keys are arranged in a systematic order and are based on diagnostic soil criteria (or criterion). In addition to the revisions described above, a number of errors and discrepancies in the 1987 edition of The Canadian System of Soil Classification have been corrected. As well, the wording has been changed in many instances to remove ambiguity, to standardize terminology, and to make the intent more easily understood.