These lecture notes intend to form a short pedagogical introduction to the
use of typical type Ia-Supernovae (hereafter SNIa) as standard candles to d
etermine the energy density of the universe. Problems of principle for taki
ng SNIa as cosmological probes are painted out, and new attempts at solving
them are indicated including the empirical width-luminosity relation (WLR)
and its possible explanations. Finally, the observations of SNIa at high r
edshift carried out by two major teams are briefly reviewed and their inter
pretation as evidence for an accelerating universe is also rapidly discusse
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