  The basic equations, concepts, and modes of linear, ideal, MHD waves -- slow,
Alfv\'en and fast -- are set out and generalised to gravitationally-stratified
atmospheres. Particular attention is devoted to mode conversion, wherein the
local behavior of a global wave changes from one mode to another in passing
through particular atmospheric layers. Exact solutions are explored where
available. Eikonal methods -- WKBJ and ray theory -- are described. Although
our emphasis is on the theoretical underpinning of the subject, the solar
atmospheric heating implications of fast/slow and fast/Alfv\'en conversions are
discussed in detail.
