Astrochemistry: the issue of molecular complexity in astrophysical
  environments
  Astrochemistry aims at studying chemical processes in astronomical
environments. This discipline -- located at the crossroad between astrophysics
and chemistry -- is rapidly evolving and explores the issue of the formation of
molecules of increasing complexity in particular physical conditions that
deviate significantly from those frequently encountered in chemistry
laboratories. The main goal of this paper is to provide an overview of this
discipline. So far, about 170 molecules have been identified in the
interstellar medium (ISM). The presence of this molecular diversity constitutes
a firm evidence that efficient formation processes are at work in the
interstellar medium. This paper aims at summarizing most of present ideas that
are explored by astrochemists to investigate the chemistry taking place in
various astronomical environments, with emphasis on the particular conditions
which are met in space (including radiation fields, cosmic-rays, low
densities...). The more ambitious question of the molecular complexity is
addressed following two approaches presented to be converging. The first
approach considers the growing complexity starting from the most simple
chemical species in interstellar environments, and the second approach
envisages successive precursors of the most complex species commonly found on
Earth, and in particular in our biochemistry. The issue of molecular complexity
constitutes one of the main modern scientific questions addressed by
astrochemistry, and it is used as a guideline across this paper.
