Angular Momentum Evolution of Young Stars and Disks

The effects of angular momentum play an important role in star formation. However, when a young star emerges on the Hertzsprung-Russell diagram it is generally rotating slowly. This article reviews the observations regarding the angular momentum of molecular cloud cores, of proto stars, and of the disks, binary orbits, and spin of young stars. Various physical processes for transfer of angular momentum during the star formation process are discussed, including magnetic braking during the molecular cloud stage; fragmentation during the protostar collapse; viscosity, magnetic torques, and gravitational torques in disks; and magnetically driven mass loss in a star-disk system. Important problems for future study are highlighted.