Astrochemistry and compositions of planetary systems
  Planets form and obtain their compositions in disks of gas and dust around
young stars. The chemical compositions of these planet-forming disks regulate
all aspects of planetary compositions from bulk elemental inventories to access
to water and reactive organics, i.e. a planet's hospitality to life and its
chemical origins. Disk chemical structures are in their turn governed by a
combination of {\it in situ} chemical processes, and inheritance of molecules
from the preceding evolutionary stages of the star formation process. In this
review we present our current understanding of the chemical processes active in
pre- and protostellar environments that set the initial conditions for disks,
and the disk chemical processes that evolve the chemical conditions during the
first million years of planet formation. We review recent observational,
laboratory and theoretical discoveries that have led to the present view of the
chemical environment within which planets form, and their effects on the
compositions of nascent planetary systems. We also discuss the many unknowns
that remain and outline some possible pathways to addressing them.
