  Planets form in disks of gas and dust around young stars. The disk molecular
reservoirs and their chemical evolution affect all aspects of planet formation,
from the coagulation of dust grains into pebbles, to the elemental and
molecular compositions of the mature planet. Disk chemistry also enables unique
probes of disk structures and dynamics, including those directly linked to
ongoing planet formation. Here we review the protoplanetary disk chemistry of
the volatile elements HOCNSP, the associated observational and theoretical
methods, and the links between disk and planet chemical compositions. Three
takeaways from this review are: (1) The disk chemical composition, including
the organic reservoirs, is set by both inheritance and in situ chemistry. (2)
Disk gas and solid O/C/N/H elemental ratios often deviate from stellar values
due to a combination of condensation of molecular carriers, chemistry, and
dynamics. (3) Chemical, physical, and dynamical processes in disks are closely
linked, which complicates disk chemistry modeling, but these links also present
an opportunity to develop chemical probes of different aspects of disk
evolution and planet formation.
