  This article presents a review on the observations and theoretical modeling
of the evaporation of extrasolar planets. The observations and the resulting
constraints on the upper atmosphere (thermosphere and exosphere) of the
"hot-Jupiters". are described. The early observations of the first discovered
transiting extrasolar planet, HD209458b, allowed the discovery that this planet
has an extended atmosphere of escaping hydrogen. Subsequent observations showed
the presence of oxygen and carbon at very high altitude. These observations
give unique constraints on the escape rate and mechanism in the atmosphere of
hot-Jupiters. The most recent Lyman-alpha HST observations of HD189733b and
MgII observations of Wasp-12b allow for the first time comparison of the
evaporation from different planets in different environments. Models to
quantify the escape rate from the measured occultation depths, and an energy
diagram to describe the evaporation state of hot-Jupiters are presented. Using
this diagram, it is shown that few already known planets like GJ876d or
CoRot-7b could be remnants of formerly giant planets.
