  The processes contributing to the evolution of an initially weak magnetic
field in a differentially rotating star are reviewed. These include rotational
smoothing (akin to convective expulsion) and a list of about 5 instabilities,
among them magnetorotational instability, byoyancy instability, and pinch-type
instabilities. The important effects of thermal and magnetic diffusion on these
instabilities are analyzed in some detail. The first instability to set in is a
pinch-type instability. It becomes important in modifying the field
configuration before magnetic buoyancy-driven instabilities set in. The
evolution of an initially strong field remains a more open question, including
the old problem whether dynamically stable magnetic equilibria exist in stars.
