Planets are formed from the disk of gas and dust around a star, but the mechanisms for doing so are imperfectly understood. Gas is the key driver in the dynamical evolution of planets, for example, because it is the dominant component of the disk (by mass). The timescale over which the gas dissipates sets the timescale for planet formation, yet its distribution in disks is just starting to be carefully measured. Similarly, the chemical composition of the gas determines the composition of the future planets and their atmospheres, but even after decades of studying protoplanetary disks, their chemical compositions are poorly constrained; even the gas-to-dust ratios are largely unknown.