The potential discovery of Earth-like planets around
other stars will need, apart from a sophisticated technological
development, the design of techniques for identifying the most
representative parameters of their atmospheres and surfaces. The
search for life will constitutes the holy grail of this search.
However, for many decades the observations will provide data with
low spatial and spectral resolution. In the meantime, the Earth will
be for us a sort of Rosetta stone to interpret the exoplanet's
spectra. Here we present a review of the main results of simulations
and observations looking at our planet with nil or low resolution.
In other words, we will study ``The Earth seen as a Exoplanet".