  This review introduces physical processes in protoplanetary disks relevant to
accretion and the initial stages of planet formation. After a brief overview of
the observational context, I introduce the elementary theory of disk structure
and evolution, review the gas-phase physics of angular momentum transport
through turbulence and disk winds, and discuss possible origins for the
episodic accretion observed in Young Stellar Objects. Turning to solids, I
review the evolution of single particles under aerodynamic forces, and describe
the conditions necessary for the development of collective gas-particle
instabilities. Observations show that disks can exhibit pronounced large-scale
structure, and I discuss the types of structures that may form from gas and
particle interactions at ice lines, vortices and zonal flows, prior to the
formation of large planetary bodies. I conclude with disk dispersal.
