The loss of material from the surface of a metal being impacted by small, solid particles carried in a flowing gas occurs by a sequence of mechanical actions. The present work, which was presented as the plenary lecture at CORROSION/95, describes the mechanisms that occur on ductile and brittle materials and relates these mechanisms to surface loss rates. The ability of these mechanisms to be represented by mathematical models is assessed. The effects of material properties on erosion behavior is discussed. Combined erosion-corrosion surface degradation mechanisms are explained.