Gamma Ray Bursts are among the most powerful astrophysical sources and they release up to 1e54 erg, if isotropic, in less than few hundred seconds. Their detection in the hard gamma ray band and out to very high redshift (z~6.3) makes them a powerful new cosmological tool (a) to study the reionization epoch, (b) to unveil the properties of the IGM, (c) to study the present universe geometry and (d) to investigate the nature and cosmic evolution of Dark Energy. While GRBs will surely help to understand the first two issues in the future, the present link between GRBs and cosmology has been made concrete by the recent discovery of tight correlations between their rest frame prompt and afterglow emission properties which allowed their use as standard candles.