One of the outstanding issues related to the physics of high-gravity pulsators is concerned with the driving mechanisms at work in these stars. I will review the status of the nonadiabatic calculations currently available and address the question of the empirical and theoretical instability regions in the HR diagram for evolved, compact stars. These include the three families of pulsating white dwarfs (g-mode pulsators excited through a mechanism associated with partial ionization and convection in the stellar envelope), the pulsating subdwarf B stars (p-mode variables excited through a classic kappa mechanism associated with the radiative levitation of iron in the stellar envelope), and the "Betsy stars", the brand new class of long-period, g-mode pulsators of the subdwarf B type discovered recently.

 
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