  As soon as it was realized that long GRBs lie at cosmological distances,
attempts have been made to use them as cosmological probes. Besides their use
as lighthouses, a task that presents mainly the technological challenge of a
rapid deep high resolution follow-up, researchers attempted to find the Holy
Grail: a way to create a standard candle from GRB observables. We discuss here
the attempts and the discovery of the Ghirlanda correlation, to date the best
method to standardize the GRB candle. Together with discussing the promises of
this method, we will underline the open issues, the required calibrations and
how to understand them and keep them under control. Even though GRB cosmology
is a field in its infancy, ongoing work and studies will clarify soon if and
how GRBs will be able to keep up to the promises.

Astrophysics