  This is a draft chapter for a book, entitled Physical Processes in
Circumstellar Disks around Young Stars, which is scheduled for publication by
the University of Chicago Press as one of its Theoretical Astrophysics Series
volumes. Sect. 1 presents the motivation for considering the effects of a
large-scale, ordered magnetic field on the formation and evolution of
protostellar disks. Sect. 2 outlines the physical principles that underlie the
magnetohydrodynamics of disks that are threaded by such a field. Sect. 3
discusses the formation and early evolution of disks that result from the
collapse of a rotating molecular cloud core that is coupled to the
insterstellar magnetic field. Sect. 4 reviews the observational evidence for
the disk--wind connection and describes the structure of magnetically
accelerated disk outflows, focusing on centrifugally driven winds; it then goes
on to discuss the equilibrium and stability properties of weakly ionized
protostellar accretion disks in which the transport of angular momentum is
dominated by a wind of this type. Sect. 5 considers the coupling between the
central protostar and the surrounding disk through the protostellar magnetic
field, covering, in turn, the phenomenology, basic concepts, and results of
numerical simulations. The chapter is summarized in Sect. 6, which also
contains a discussion of future research directions.
