  The Hubble Space Telescope (HST) detection of an extrasolar planet atmosphere
in 2001 was a landmark step forward for the characterization of extrasolar
planets. HST detected the trace element sodium, via the neutral atomic
resonance doublet at 593 nm, in the transiting extrasolar giant planet
HD209458b. In this paper I discuss the significance of this first ever
extrasolar planet atmosphere detection. I explain how the sodium measurement
can be used as a constraint on HD209458b atmosphere models and review recent
interpretations of the lower-than-expected sodium line strength.
