  We review all the models proposed for the progenitor systems of Type Ia
supernovae and discuss the strengths and weaknesses of each scenario when
confronted with observations. We show that all scenarios encounter at least a
few serious diffculties, if taken to represent a comprehensive model for the
progenitors of all Type Ia supernovae (SNe Ia). Consequently, we tentatively
conclude that there is probably more than one channel leading SNe Ia. While the
single-degenerate scenario (in which a single white dwarf accretes mass from a
normal stellar companion) has been studied in some detail, the other scenarios
will need a similar level of scrutiny before any firm conclusions can be drawn.
