Ballooning theory is an asymptotic method for treating modes with short wavelength across the magnetic field lines but essentially no phase variation along the lines. The latter constraint leads to problems with maintaining the physically required poloidal and toroidal periodicity, which can be solved in various ways. In this paper we review the approach of Dewar and Glasser, which applies to general three-dimensional equilibria, and also review some recent applications of the formalism to a heliotron/torsatron test case.