  Protoplanetary disks are the birthplaces of planetary systems. The evolution
of the star-disk system and the disk chemical composition determines the
initial conditions for planet formation. Therefore a comprehensive
understanding of the main physical and chemical processes in disks is crucial
for our understanding of planet formation. We give an overview of the early
evolution of disks, discuss the importance of the stellar high-energy radiation
for disk evolution and describe the general thermal and chemical structure of
disks. Finally we provide an overview of observational tracers of the gas
component and disk winds.
