Present theories of the origin of white dwarfs are discussed; it is shown that all theories imply that there can be no effective energy sources present in a white dwarf at the time of its birth. The temperature distribution of a white dwarf is then discussed on the assumption that no energy liberation occurs within the star, and that it radiates at the expense of the thermal energy of the heavy particles present. In the resulting picture, a white dwarf consists of a degenerate core containing the bulk of the mass, surrounded by a thin, non-degenerate envelope. The energy flow in the core is due to the large conductivity of the degenerate electrons, while the high opacity of the outer layer keeps down the luminosity to a low level. Estimates of the ages of observed white dwarfs are giyen and interpreted. Finally, it is shown that white dwarfs may accrete energy sources and yet continue to cool off, provided the temperature at the time of accretion is not too high; this suggests a possible model for Sirius B.