This result seems in contradiction with the recent observations of Type Ia SN rate in the Great Observatories Origins Deep Survey (GOODS) fields: at high redshift rates are best fit by the presence of long delay times between progenitor formation and SN explosion. This discrepancy is only apparent: both results can be explained by assuming that Type Ia SNe originate from two different populations, characterized by very different delay times: about half of the SNe Ia derive from a relatively young stellar population (fast exploders), the other half comes from old stellar populations having delay times of 2—4 Gyrs (slow exploders). Since the two populations dominate at different redshifts, the use of Ia SNe as cosmological standard candles can be questioned. Details are given in two papers by Mannucci et al. (2004a,b).