Trace elements in the photospheres of hot white dwarf stars

X-ray, ultraviolet, and optical observations of hot white dwarfs have demonstrated that a number of these objects contain observable quantities of trace elements. This paper reviews recent observational results and includes some discussion of the possible roles of pre-white dwarf evolution, levitation of trace elements by radiation pressure, and accretion in producing the peculiar patterns of trace element abundances in these stars. 30 references