The majority of solar structure and phenomena are a result of the interaction of the solar plasma with the magnetic field (Osterbrock 1961). Such interactions are governed by a set of equations known collectively as magnetohydrodynamics (MHD). These are a combination of the equations of electric and magnetic fields, and those of fluid dynamics. Using simple approximations and combining linearised versions of these equations leads to wave dispersion relationships which describe the behaviour of a plasma in a magnetic field. This chapter contains a brief summary of these equations and the properties of the wave modes which arise from them in both a homogeneous and inhomogeneous plasma.