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Scalar mesons are a key expression of the infrared regime of QCD. Thelightest of these is the $\sigma$. Now that its pole in the complex energyplane has been precisely located, we can ask whether this state is transiently${\bar q}q$ or ${\bar {qq}} qq$ or a multi-meson molecule or largely glue? Thetwo photon decay of the $\sigma$ can, in principle, discriminate between thesepossibilities. We review here how the $\gamma\gamma\to\pi^+\pi^-$, $\pi^0\pi^0$cross-sections can be accurately computed. The result not only agrees withexperiment, but definitively fixes the radiative coupling of the $\sigma$. Thisequates to a two photon width of $(4.1 \pm 0.3)$ keV, which accords with thesimple non-relativistic quark model expectation for a ${\bar u}u, {\bar d}d$scalar. Nevertheless, robust predictions from relativistic strong coupling QCDare required for each of the possible compositions before we can be sure whichone really delivers the determined $\gamma\gamma$ coupling.

Location, correlation, radiation: where is the $\sigma$, what is its structure and what is its coupling to photons?