  Quasars are the most luminous sources in the Universe. They are currently
observed out to redshift z ~ 7 when the Universe was less than one tenth of its
present age. Since their discovery 50 years ago astronomers have dreamed of
using them as standard candles. Unfortunately quasars cover a very large range
(8 dex) of luminosity making them far from standard. We briefly review several
methods that can potentially exploit quasars properties and allow us to obtain
useful constraints on principal cosmological parameters. Using our 4D
Eigenvector 1 formalism we have found a way to effectively isolate quasars
radiating near the Eddington limit. If the Eddington ratio is known, under
several assumptions it is possible to derive distance independent luminosities.
We discuss and the main statistical and systematic errors involved, and whether
these "standard Eddington candles" can be actually used to constrain
cosmological models.
