The theory of ballooning modes, which are models localized to a particular magnetic field line, is extended to configurations relevant to space plasmas. Included are the effects of gravity and rotation and, in particular, boundary effects on magnetic field lines which intersect the plasma boundary. Three types of boundary conditions are considered, corresponding to perfectly conducting, conducting, and insulating boundaries. The interchange instability is also examined and is shown to be a special case of the balooning instability. ¿1991 American Geophysical Union