  Protoplanetary disks (PPDs) surrounding young stars are short-lived (~0.3-10
Myr), compact (~10-1000 AU) rotating reservoirs of gas and dust. PPDs are
believed to be birthplaces of planetary systems, where tiny grains are
assembled into pebbles, then rocks, planetesimals, and eventually planets,
asteroids, and comets. Strong variations of physical conditions (temperature,
density, ionization rate, UV/X-rays intensities) make a variety of chemical
processes active in disks, producing simple molecules in the gas phase and
complex polyatomic (organic) species on the surfaces of dust particles. In this
entry, we summarize the major modern observational methods and theoretical
paradigms used to investigate disk chemical composition and evolution, and
present the most important results. Future research directions that will become
possible with the advent of the Atacama Large Millimeter Array (ALMA) and other
forthcoming observational facilities are also discussed.
