  Pions were predicted by H. Yukawa as force carriers of the inter-nucleon
forces, and were detected in 1947. Today they are known to be bound states of
quarks and anti-quarks of the two lightest flavours. They satisfy Bose
statistics, and are the lightest particles of the strong interaction spectrum.
Determination of the parameters of the Standard Model, including the masses of
the lightest quarks, has only recently reached high precision on the lattice.
Pions are also known to be pseudo-Goldstone bosons of spontaneously broken
approximate axial-vector symmetries, and a probe of their properties and
interactions at high precision tests our knowledge of the strong interactions.
While also being a probe of the solution of the strong interactions on the
computer, which is known as lattice gauge theory. Despite their long history,
there are significant experimental and theoretical challenges in determining
their properties at high precision. Examples include the lifetime of the
neutral pion, and the status of their masses and decay widths in effective
field theories. Pion-pion scattering has been studied for several decades using
general methods of field theory such as dispersion relations based on
analyticity, unitarity and crossing. Knowledge from these theoretical methods
are used to confront high precision experimental data, and to analyze them to
extract information on their scattering and phase shift parameters. This
knowledge is crucial for estimating the Standard Model contributions to the
anomalous magnetic moment of the muon, which is being probed at Fermilab in
ongoing experiments. Other sensitive tests include the rare decay of the eta
meson into three pions, which represents an isospin violating decay. The
present article briefly reviews these important developments.
