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The low $Q^2$ slopes of the the transition form factors provide a uniquemethod to measure the sizes of the neutral pseudo-scalar mesons, since they donot have electromagnetic form factors. From the slope one obtains the "axialtransition RMS radius" $ R_{PS,A} = \sqrt{<r^2>}$ for each PS meson. Thepresent status of theory and experiment for these quantities are presented. Acomparison of the $ R_{PS,A}$ is presented along with the electromagnetic andscalar radii of the $\pi^{\pm}$ mesons and the proton. We observe the strikingsimilarity of the values of axial transition radii of all of the pseudoscalarmesons to each other and to the charge radius of the $\pi^{\pm}$. In the $Q^2$= 0 limit the transition form factor is a measure of the pseudo-scalar mesonradiative width (lifetime) and is a possible fourth (unexploited) method toperform such a measurement. The $\pi^{0} \rightarrow \gamma \gamma$ decay rateis a test of QCD at the confinement scale. There is a firm QCD prediction witha theoretical uncertainty of $\simeq $ 1 \% which calls for an experimentaltest at the same level of accuracy. There are three methods that have beenutilized to perform this measurement and the present status of the experimentaltests are outlined. The current accuracy is significantly less than thetheoretical uncertainty. The efforts to improve this are briefly summarized.

The pi0, eta, eta' -> gamma gamma*(Q^2) Decay Rates and Radii