  Gamma Ray Bursts (GRBs) are among the most powerful sources in the Universe:
they emit up to 10^54 erg in the hard X-ray band in few tens of seconds. The
cosmological origin of GRBs has been confirmed by several spectroscopic
measurements of their redshifts, distributed in the range 0.1-6.3. These two
properties make GRBs very appealing to investigate the far Universe. The
energetics implied by the observed fluences and redshifts span at least four
orders of magnitudes. Therefore, at first sight, GRBs are all but standard
candles. But there are correlations among some observed quantities which allow
us to know the total energy or the peak luminosity emitted by a specific burst
with a great accuracy. Through these correlations, GRBs become "known" candles
to constrain the cosmological parameters. One of these correlation is between
the rest frame peak spectral energy E_peak and the total energy emitted in
gamma--rays E_gamma, properly corrected for the collimation factor. Another
correlation, discovered very recently, relates the total GRB luminosity L_iso,
its peak spectral energy E_peak and a characteristic timescale T_0.45, related
to the variability of the prompt emission. It is based only on prompt emission
properties, it is completely phenomenological, model independent and
assumption--free. The constraints found through these correlations on the
Omega_M and Omega_Lambda parameters are consistent with the concordance model.
The present limited sample of bursts and the lack of low redshift events,
necessary to calibrate these correlations, makes the cosmological constraints
obtained with GRBs still large compared to those obtained with other
cosmological probes (e.g. SNIa or CMB). However, the newly born field of
GRB--cosmology is very promising for the future.
