Abstract In this chapter we review recent advances in our understanding of the chemical and isotopic evolution of protoplanetary disks and the solar nebula. Current observational and meteoritic constraints on physical conditions and chemical composition of gas and dust in these systems are presented. A variety of chemical and photochemical processes that occur in planet-forming zones and beyond, both in the gas phase and on grain surfaces, are overviewed. The discussion is based upon radio-interferometric, meteoritic, space-borne, and laboratory-based observations, measurements and theories. Linkage between cosmochemical and astrochemical data are presented, and interesting research puzzles are discussed.

Circumstellar disks surrounding young low-and intermediate-mass stars and brown dwarfs possess sufficient material to accrete planetary systems (e.g. Lin & Papaloizou 1980; Lissauer 1987; Payne & Lodato 2007). The chemical composition of the constituent gas and the inherent physical properties of dust regulate the mechanism and timescale for planetary formation. Molecules and dust are significant heating and cooling agents of the gas connected through collisions (see Chapter 3), while dust grains also determine the opacity of the medium. Ionization that allows the coupling of the gas to magnetic fields is regulated by photochemical and molecular reactions and magneto-hydrodynamical processes. To develop suitable evolutionary models of the solar nebula, a variety of relevant physical and chemical processes must be identified and resolved.