Chapter 3 introduces physical processes that lead to the evolution of gaseous protoplanetary disks. It begins with a derivation of the equation describing the evolution of a thin viscous accretion disk, a discussion of solutions, and introduction of the Shakura-Sunyaev alpha prescription. Hydrodynamic sources of angular momentum transport, including self-gravity, the vertical shear instability, and vortices, are discussed. Magnetohydrodynamic (MHD) sources of angular momentum transport are reviewed, starting with the magnetorotational instability in ideal MHD. The non-ideal induction equation of MHD is derived, and the importance of Ohmic diffusion, ambipolar diffusion, and the Hall effect for protoplanetary disks is reviewed. A simple model for angular momentum loss due to a magnetized disk wind is discussed. The chapter concludes with a description of disk dispersal via photoevaporation, and magnetospheric star-disk interaction.