The effects of core crystallization and of convection in the envelope on the cooling of white dwarfs are reviewed. The case of a 0.6 M⊙ white dwarf composed of an oxygen core and a helium envelope is taken as example. Also the amount of hydrogen that a white dwarf can accrete before nuclear burning occurs is estimated and possible evolutionary relations between white dwarfs of types DA, DB and DC, as advanced by Baglin and Vauclair, are presented.