Of all stars in the Hertzprung-Russell diagram, white dwarfs are those for which the clues on past evolution given by photospheric abundances are probably the hardest to decipher. This is because the cooling phase of white dwarfs, a relatively uneventful phase from an evolutionary point of view, is, in contrast, a most active phase for the evolution of the chemical composition of the envelope. Indeed, it is now well established that the often puzzling variety of surface abundances observed in white dwarf stars can be traced to the simultaneous operation, in the outer layers of these stars, of a variety of physical processes which will also erase the abundances present in the photosphere at the onset of cooling.
Downward element diffusion in the intense gravitational field of the degenerate star is perhaps the mechanism which is the most closely identified with white dwarf stars. However, convective mixing, ordinary diffusion, radiative forces, winds, and accretion from the interstellar medium all are equally important processes which, at times, compete efficiently with the rapid element segregation expected in those stars. The various regions, along the cooling sequence of white dwarfs, where individual processes are expected to operate, have been summarized by Fontaine and Wesemael (1987). We illustrate here various combinations of these mechanisms which have been found in white dwarfs, and show how their competition affects the observed abundance patterns. The unity underlying these cases stems from the fact that, in many cases, progress in investigating these complicated situations has come only through the combination of evolutionary calculations with new and powerful numeriques techniques which have been developed at Montréal (Pelletier 1986; Pelletier, Fontaine, and Wesemael 1989).