There are at least two types of soils that contain gypsum base upon the source of gypsum.  Gypsiferous soils are those soils that have a gypsic horizon with the gypsum inherited from the parent material or deposited as dust.  Gypseous soils are soils with gypsic horizons formed from gypsum-rich parent material, i.e. gypsum constitutes the majority of the soil material from which the soil developed.  The majority of mapped soil containing gypsum are gypsiferous soils and these soils seem to fit nicely into the various orders of Soil Taxonomy.  Gypseous soils are of large extent is Texas and New Mexico, but have not been extensively studied. There are several mapping, laboratory and classifications question that are difficult because gypsum as a parent material is unique from soils primarily derived from silicate mineral assemblages. What makes soils with gypsum unique and what do we do differently when we sample, characterize and classify gypseous soils? There are at least a dozen unique characteristics.  I will focus on the physical and mineralogical uniqueness and how these properties can be used to assist both the field soils scientists for mapping, interpretations and classification.