Pulsations in white dwarf stars

I will present a description of the six distinct families of pulsating white dwarfs that are currently known. Pulsations are present at various stages of the evolution (from hot, pre-white dwarfs to cool white dwarfs), at various stellar masses, and for various atmospheric compositions. In all of them, a mechanism linked to opacity changes along the evolution drives the oscillations. The existence of these oscillations offers the opportunity to apply asteroseismology for constraining physics inside white dwarfs. The direct comparison between observed and theoretical pulsation frequencies yields the global parameters (e.g. mass and radius) and internal structure and composition (e.g. envelope layering, core composition) of the star. The deep understanding of the driving mechanism provides stringent constraints on the physical conditions at work where oscillations are excited. I will present the major achievements in the field of white dwarf asteroseismology, as well as the needed improvements for coming years in this field of research.