The Horsehead nebula, a template source for interstellar physics and
  chemistry
  We present a summary of our previous investigations of the physical and
chemical structure of the Horsehead nebula, and discuss how these studies led
to advances on the understanding of the impact of FUV radiation on the
structure of dense interstellar clouds. Specific molecular tracers can be used
to isolate different environments, that are more sensitive to changes in the
FUV radiation or density than the classical tracers of molecular gas : the CO
isotopologues or the dust (sub)millimeter continuum emission. They include the
HCO or CCH radicals for the FUV illuminated interfaces, or the molecular ions
H$^{13}$CO$^+$, DCO$^+$ and other deuterated species (DNC, DCN) for the cold
dense core. We discuss future prospects in the context of Herschel and ALMA.
