  We present a relativistic calculation of two-photon decays for heavy and
light mesons in the framework of the Salpeter equation for quark-antiquark
states. The meson-photon-photon vertex is computed by reconstructing the
Bethe-Salpeter vertex function and evaluating the four-dimensional Feynman
diagram with off-shell quark amplitudes. The two-photon width for light and
heavy quarkonia up to spin equal six are calculated with different parameter
sets taken from the literature thus giving a complete overview on the mesonic
two-photon physics. We find that relativistic effects including the negative
energy components of the wave function are important for any two-photon width -
even for heavy quarkonia - yielding a remarkable agreement with available data.
