Type Ia supernovae play a key role in the determination of the
cosmological parameters. Despite
several decades of observations and theoretical studies, the nature of
SN Ia progenitors is still unknown, and this fact brings ambiguity
into the interpretation of the cosmological results. One of the
most promising approaches to clarify the nature of SN Ia progenitors
consists in studying the delay time distribution between the
formation of the progenitor star and its explosion as a SN.
We will show how recent observations of the evolution of the SN Ia rate
with redshift, the dependence of the SN Ia rate on the colours of
the parent galaxies, and the enhancement of the SN
Ia rate in radio-loud early-type galaxies, can be used to
derive, for the first time on robust empirical grounds, the delay
time distribution.  The results support the hypothesis
that SNe Ia are produced by two different populations of progenitors,
each one dominating at different redshifts.
This effet has important consequences on the chemical evolution of
galaxies and IGM.