  The two-photon decay of heavy, helium-like ions is investigated based on
second-order perturbation theory and Dirac's relativistic equation. Special
attention has been paid to the angular emission of the two photons, i.e., how
the angular correlation function depends on the shell structure of the ions in
their initial and final states. Moreover, the effects from the (electric and
magnetic) non-dipole terms in the expansion of the electron-photon interaction
are discussed. Detailed calculations have been carried out for the two-photon
decay of the $1s2s \, ^1S_0$, $1s2s \, ^3S_1$ and $1s2p \, ^3P_0$ states of
helium-like xenon Xe$^{52+}$, gold Au$^{77+}$ and uranium U$^{90+}$ ions.
