The two most commonly used soil classification systems in the world are the World Reference Base (WRB) developed in a joint effort by the Food and Agriculture Organization of the United Nations, the International Union of Soil Sciences, and the ISRIC International Soil and Reference Information Centre and Soil Taxonomy (ST) developed by the United States Department of Agriculture. These systems have many similarly (but not identically) named and defined diagnostic horizons and features. This paper presents the diagnostic horizons, properties, and materials used in the WRB and the counterpart horizons and characteristics diagnostic for the higher categories from ST. We rated the diagnostic criteria as identical, very similar, similar or different between the two systems as of 1999. Despite the fact that both systems are attempting to equate to the same soil-forming processes only one diagnostic (organic soil materials) was named and defined identically in both systems. There were 8 diagnostics used in the WRB system that had very similar criteria and an additional 17 that had similar criteria to ST. These 25 diagnostics could be made identical with minimal changes. Fourteen of the diagnostic horizons of WRB had criteria that are different than those used in ST. Additionally, 10 of the diagnostic horizons of WRB had no counterpart in ST and 4 diagnostic horizons of ST had no counterpart in WRB. We then compare the 2006 versions of both WRB and ST and conclude with a summary of extent to which the diagnostics of the two systems have been harmonized in the 2006 editions.