  Type Ia supernovae are thought to be the outcome of the thermonuclear
explosion of a white dwarf in a close binary system. Two possible scenarios,
not necessarily incompatible, have been advanced. One assumes a white dwarf
that accretes matter from a nondegenerate companion (the single degenerate
scenario), the other assumes two white dwarfs that merge as a consequence of
the emission of gravitational waves (the double degenerate scenario). The delay
time distribution of star formation bursts strongly suggests that the DD
scenario should be responsible of the late time explosions, but this
contradicts the common wisdom that the outcome of the merging of two white
dwarfs is an accretion induced collapse to a neutron star. In this contribution
we review some of the most controversial issues of this problem.
