Astrochemistry in external galaxies: how to use molecules as probes of
  their physical conditions
  It is now well established that chemistry in external galaxies is rich and
complex. In this review I will explore whether one can use molecular emissions
to determine their physical conditions. There are several considerations to
bear in mind when using molecular emission, and in particular molecular ratios,
to determine the densities, temperatures and energetics of a galaxy, which I
will briefly summarise here. I will then present an example of a study that
uses multiple chemical and radiative transfer analyses in order to tackle the
too often neglected `degeneracies' implicit in the interpretation of molecular
ratios and show that only via such analyses combined with multi-species and
multi-lines high spatial resolution data one can truly make molecules into
powerful diagnostics of the evolution and distribution of molecular gas.
