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