[en] The stability of toroidal plasmas is often described by approximate MHD equations for a small number of scalar functions (reduced MHD equations). We propose a general representation of toroidal MHD in terms of stream-functions and potentials, which facilitates the physical interpretation of various terms and therefore is well suited for deriving different types of reduced equations. We qualitatively discuss stationary plasma states bifurcating from a given equilibrium, and we treat the particular case of a MHD-unstable cylindrical equilibrium