  The electric ${\alpha}_{\pi}$ and magnetic ${\beta}_{\pi}$ charged pion
Compton polarizabilities are of fundamental interest in the low-energy sector
of quantum chromodynamics (QCD). They characterize the induced dipole moments
of the pion during ${\gamma}{\pi}$ Compton scattering. Pion polarizabilities
affect the shape of the ${\gamma}{\pi}$ Compton scattering angular
distribution.The combination $({\alpha}_{\pi} - {\beta}_{\pi})$ was measured
by: (1) CERN COMPASS via ${\pi} Z \rightarrow {\pi} Z {\gamma}$ radiative pion
Primakoff scattering in the nuclear Coulomb field, (2) SLAC PEP Mark-II via
two-photon production of pion pairs, ${\gamma} {\gamma} \rightarrow {\pi}^+
{\pi}^-$, and (3) Mainz Microtron MAMI via radiative pion photoproduction from
the proton, ${\gamma} p \rightarrow {\gamma} {\pi}^+ n$. Ongoing and planned
pion polarizability experiments (CERN COMPASS, BESIII at Beijing, JLab at
Newport News) are also described. The Mark-II pion polarizability 95%
confidence interval is approximately $0-11 \times 10^{-4} {fm}^{3}$, based on
$({\alpha}_{\pi} - {\beta}_{\pi}) = (4.4 \pm 3.2) \times 10^{-4} {fm}^{3}$. The
Mainz value $({\alpha}_{\pi} - {\beta}_{\pi}) = 11.6 \times 10^{-4} {fm}^{3}$
is excluded on the basis of a dispersion relations calculation which uses the
Mainz value as input, and gives significantly too large ${\gamma} {\gamma}
\rightarrow {\pi}^{0} {\pi}^{0}$ cross sections compared to DESY Crystal Ball
data. To date, only the COMPASS polarizability measurement has acceptably small
uncertainties. Its value $({\alpha}_{\pi} - {\beta}_{\pi}) = (4.0 \pm 1.8)
\times 10^{-4} {fm}^{3}$ agrees well with the two-loop ChPT prediction
$({\alpha}_{\pi} - {\beta}_{\pi}) = (5.7 \pm 1.0) \times 10^{-4} {fm}^{3}$,
thereby strengthening the identification of the pion with the Goldstone boson
of QCD.
